@antonyh
Welcome! I tend to use the web interface from every platform I use (Linux desktop, Windows desktop, Android), and I find that it works better than any of the apps with a few exceptions like posting GIFs.
Infosec.exchange is the best-run instance on the Fediverse, in my humble opinion. There aren't a whole lot of unwritten rules, but please don't do be a jerk or do anything that makes our illustrious leader @jerry's life more difficult. Our instance is quite a bit less dogmatic than many other instances, but broadly speaking the broader Mastodon/Fediverse is notorious for being the only social media network with its own HOA, so you can expect people (mostly from other instances) to share their... opinions. You can mostly ignore them, and the block/mute button is your friend.
As far as follow recommendations, it kinda depends on what you're looking for. My own interests tend to focus on general infosec, cyber threat intel, US/world news, geopolitics (e.g., Ukraine, US politics), niche interesting tech, and weird shitposters. I recommend finding one or two active users whose interests align with yours, follow them, pivot into the accounts they retoot, follow them, and just grow it organically.
Tip your server if you can.
Patreon: https://www.patreon.com/infosecexchange
Ko-Fi: https://ko-fi.com/infosecexchange
Liberpay: https://liberapay.com/Infosec.exchange/