Good summary! There are however apps that sum up the points of your comments and posts and show them on your profile. This is only accurate for your own profile , as posts of others on other instances are not guaranteed to federate and taken into account.
Another good one is commonvoice.mozilla.org/en/ where you either read out text or validate recordings by others. These samples help creating artificial voices.
Absolutely. This has even more impact, but also a higher learning curve (OP asked to answer questions). I contribute on HOTOSM as well, it is an amazing project!
Great that you are looking for a way to contribute to something meaningful. OpenStreetMap aims to create a free map of the world, at detail levels nobody else does. Besides the obvious, it helps groups that are marginalized (how do I navigate as a blind person? Where are safe spots to cross the street? Does this restaurant not only have a ramp for my wheelchair, but does it also have restrooms that accommodate for my needs?)
The simplest way to start is to contribute on pic4review.pavie.info - here you can basically look at pictures other people have taken, answer some questions and thus add data to OpenStreetMap.
Seems like nitter still works. Interesting how everyone, YouTube, Twitter, Reddit seem to try to to squeeze more money out of their platforms by making themselves shittier.