[Ulvinn]

I've been running into various bugs in 18 nightlies, I also have some suggestions to possible improvements (if these are welcomed by developers from a "new" forum user). is there a preferred thread to post about kodi 18 development?

[Klojum]

You can set a prefix on the left next to the subject when opening a new thread.
Use the Kodi 18 prefix.

[DarrenHill]

Depending on quite what you're working on or want to talk about, there's either the kodi development section for work on the core (but that's for true code suggestions and pull requests, not bug reports or feature suggestions), or many people just use the relevant section of the general support area, especially if it's for something like Milhouse's builds of LibreElec.

There isn't a specific thread as even though it's alpha software it's still quite widely being used and tested by various people, both team members and the more general userbase of developers and interested parties.

[jjd-uk]

Just post your issues in the General Support section using the relevant subforum for the OS you have Kodi installed on, and it's helpful if you use the Kodi 18 prefix option as Klojum has pointed out.