Here are some links to help you pick your instance (give you an idea of what behaviour is expected, if there are any subject specialisations, etc)
|gamemaking.social||My game-making related one.|
|mastodonti.co||Italian. Not sure about the rules.|
|social.chinwag.org||Australian. Not sure about the rules.|
|ausglam.space||Australian Galleries, Libraries, Archives, Museums, and records people.|
|castling.club||Play chess over ActivityPub 👌|
For using multiple accounts I like Subway Tooter. It's open-source, but not on F-Droid. I think it's mostly by Japanese developers, so you should considering submitting translations if you notice any oddities. I've done it before via Github and it was easy if you're familiar with the web interface.
MastoLab is another popular, multi-account one (open-source and on F-Droid), but I had trouble using multiple accounts that shared a domain.
I don't actually use these, but they're a helpful resource none-the-less.
It's worth checking the #mastoAdmin tag for updates too.
I keep a repo of the CSS changes I make to gm.s at: ryliejamesthomas/mastodon-custom.css-gms
This bozo made a handy list of CSS selectors and HTML elements they effect: Using Custom CSS with Mastodon. I don't approve of them personally though.
So far I've just setup a bot that posts once a day if a Youtube channel is updated. Wasn't too hard!
You can check it out at https://gamemaking.social/@classicsofgame
I used Feed2toot, which uses Python, and takes care of registering itself (bots/apps need to register to get an ID and stuff). Here're the steps I took.
Mine's hosted on my namecheap shared server. To set it up there you:
&&to switch into the virtual environment, change to the directory I want, then run the script pointing to the location of my .INI file.