There are many ways to view learning philosophy and making unbounded progress. A Popperian view is it's problem solving. You solve one problem then move on to another (that's better in some way), and keep going forever. Popper saw progress as progressing from problem to problem.
Another view, instead of finding a succession of problems, is finding a succession of things to practice. If you aren't practicing, that usually means you aren't doing effective learning. And people often have trouble figuring out what to practice for learning about abstract issues like philosophical concepts.
So one of the main things progress looks like is practicing something, then practicing something else, then practicing something else that seems more advanced or better in some way. There are other steps too besides practice, but if no practicing is taking place then the person probably isn't making progress.
Practice doesn't have to be explicit or intentional to work. People may read books for fun, without viewing it as practice, but it's still reading practice that results in them being a better reader. When practice isn't intentional, it's more common to stop making progress at some point. New readers improve when they read, but experienced readers often stop improving just from reading more books without trying to practice. If they were trying to practice, they'd make some adjustments so they could learn something new, rather than just repeating the same things over and over. The first 100 times you do something is often good practice, but you might not learn anything new on your 1000th time. Doing the same activity becomes repetitive rather than educational after a while. When people are intentionally practicing, they move on to new activities instead of repeating the same thing endlessly.
Even if practice wasn't intentional, you can still identify it as practice in retrospect. If someone asked whether you ever practiced reading as an adult, you could say "not intentionally, but I have read over 100 books" instead of saying "no". If you're listing what philosophy-relevant skills you've practiced, you could include reading. You could analyze what you got better at while reading books as an adult.
I don't think it's very hard to find some things to practice to help you make philosophical progress (so when people don't do it, I tend to think they don't want to). Options include writing, reading, text analysis and other language skills. Or logic and math skills. Or details like nested quoting. Or working on trees and other diagrams. Or you can just take philosophical concepts and practice using them.
I wrote Practice Thinking in Terms of Error Correction. You can do something similar with other concepts. For example, you can use the concept of freedom, and look at various example scenarios and look for ways they involve freedom or lack of freedom. Or you can look around for examples of people being open or closed to rational debate. You can practice learning definitions of philosophical terms with flash cards. You can look for manipulative words and actions that gain or lose social status. You can practice coming up with responses to social manipulations. After you practice responding to social manipulations by yourself with examples from movies and Reddit, you could get in regular conversations with people, who don't know that you're practicing, and try to identify when they socially attack you and try to effectively defend yourself – that way you see if your responses actually work well in real situations.
You can take a concept, say fallibilism, evolution, transparency, honesty, objectivity, bias, decisive arguments, bottlenecks or breakpoints, and brainstorm scenarios it applies to. And brainstorm scenarios it doesn't apply to. And take scenarios and then figure out if it applies. You could make yourself worksheets, with scenarios, and go through them. If you get fast, it's a sign you're automatizing the knowledge, just like with arithmetic worksheets. If you're not able to identify when a concept is relevant quickly, you'll probably miss it when the concept comes up in your regular life – you won't know when to stop and take your time thinking about whether that concept is relevant, so the opportunity will pass you by.
Another problem people have is knowing dozens of philosophical concepts and thinking they're all relevant to many things. This leads to missed opportunities because it'd take too long to use every concept every time. They need to get better at figuring out what's most relevant or most important, and learn to focus on key concepts and issues. Saying everything is relevant when it's only slightly relevant reminds me of the people who claim they're interested in basically everything, including dozens of things they'll never do, because they consider their interest level to be above zero. For philosophical concepts, people need to learn to identify significant, important relevance – so it's worth some time and effort to consider – not just above-zero relevance.
Identifying when a concept comes up as relevant is just one part of the issue. You also need to know what to do with the concept. And you need to know why the concept is right. And you need to know how to address objections, concerns and problems that come up when you apply the concept. If the concept says to do X instead of Y, you'll also need to respond to all your reasons for doing Y in ways that all parts of you are satisfied with, or else you'll have an unresolved conflict. You can practice each of these things: applying a concept, explaining the concept, defending the concept against criticism, and criticizing alternatives that contradict the concept. Each of those can be practiced individually with many different concepts.
In order to practice something, you need at least one activity to do and some way to judge success or failure. Practice involves doing actions, judging whether you succeeded, and doing them more. The basic goal is to increase your success rate until it's high. When you fail, you try to make changes next time. It's also good, early on, to just try variations to see the results. Even if you could succeed every time, you'll learn more if you push the limits and find what changes you can make and still succeed, and what changes cause failure. If you play it safe every time, you'll just know one way to do it successfully instead of figuring out where the breakpoints are that determine success or failure.
The way to get started practicing philosophy is to try a variety of relevant things that seem easy until you find some you can succeed at. Your initial goal should be to succeed as quickly and reliably as you can. You need to establish some history of success. Then you can expand on it. You should also keep exploring – trying to do variety of things to find more that you're already good at. You want to figure out what you already know, confirm that you know it by successfully doing practice activities (and creating and passing tests you set for yourself), and then make progress by learning new things that build on what you already know. You should build in small steps. Trying to do a lot of new stuff at once, so it's hard, is a way to get stuck and fail, not a way to accelerate progress (nor a way to impress anyone wise). When making progress seems hard, that's a sign your approach is wrong. Progress shouldn't be hard. Life isn't about overcoming as much adversity as you can. It's better to avoid adversity in most cases, and only occasionally face it when there's a good reason. You'll make way more progress, more quickly, if you succeed at some incremental progress every day than if you spend a month trying to do something you find difficult.