If you want to level up your game, find a new job, or grow into a new role, by all means take a course or training on your own time. All of the concerns that you listed are probably worth spending dedicated time to upskill on.
If you stay in this field for much longer, you're going to run into a lot of cases where the thing you've been doing is replaced with the New Thing. The New Thing will have a couple new ideas, but will also fundamentally handle the same concerns as your Old Thing, often in similar ways. Don't spend your free time chasing the New Thing without getting something out of it - getting paid, making a project that you wanted to make anyways, contributing to New Thing open source projects.
If you sink work into the New Thing without anyone willing to pay for it, that's fine, but it means that you might never get someone to pay for it. Most companies are more than willing to hire experienced Old Thing devs on New Thing jobs, and will give you some time to skill up.
Polling is certainly useful, but at some point introducing reliability degrades effectiveness. I certainly want to know if the app is unreachable over the open internet, and I absolutely need to know if a partner's API is down.
Wherever possible, this is a good idea. The campsite rule - tests don't touch data they didn't bring with them - helps as well.
However, many end to end tests exist as a pipeline, especially for entities that are core to the business function of the app. Cramming all sequentiality into single tests will give you all the problems described, but in a giant single test that you need to fish out the result for.
My experience with E2E testing is that the tools and methods necessary to test a complex app are flaky. Waits, checks for text or selectors and custom form field navigation all need careful balancing to make the test effective. On top of this, there is frequently a sequentiality to E2E tests that causes these failures to multiply in frequency, as you're at the mercy of not just the worst test, but the product of every test in sequence.
I agree that the tests cause less flakiness in the app itself, but I have found smokes inherently flaky in a way that unit and integration tests are not.
My team has just decided to make working smokes a mandatory part of merging a PR. If the smokes don't work on your branch, it doesn't merge to main. I'm somewhat conflicted - on one hand, we had frequent breaks in the smokes that developers didn't fix, including ones that represented real production issues. On the other, smokes can fail for no reason and are time consuming to run.
We use playwright, running on github actions. The default free tier runner has been awful, and we're moving to larger runners on the platform. We have a retry policy on any smokes that need to run in a step by step order, and we aggressively prune and remove smokes that frequently fail or don't test for real issues.
Add the new code path/behavior. Release - this can be a minor version in semver.
Mark the old code path or behavior as deprecated. Release - this can be another minor version.
In between here, clean up any dependencies or give your users time to clean up.
Remove the old code path or behavior. Release. If you're using semver, this is the major version change.
This is a stable way to make changes on any system that has a dependency on another platform, repository, or system. It's good practice for anything on the web, as users may have logged in or long running sessions, and it works for systems that call each other and get released on different cadences.
If you want to level up your game, find a new job, or grow into a new role, by all means take a course or training on your own time. All of the concerns that you listed are probably worth spending dedicated time to upskill on.
If you stay in this field for much longer, you're going to run into a lot of cases where the thing you've been doing is replaced with the New Thing. The New Thing will have a couple new ideas, but will also fundamentally handle the same concerns as your Old Thing, often in similar ways. Don't spend your free time chasing the New Thing without getting something out of it - getting paid, making a project that you wanted to make anyways, contributing to New Thing open source projects.
If you sink work into the New Thing without anyone willing to pay for it, that's fine, but it means that you might never get someone to pay for it. Most companies are more than willing to hire experienced Old Thing devs on New Thing jobs, and will give you some time to skill up.