I'm going to play this with friends on Friday and noticed it is on sale here but not on Steam at the moment. If I get it here but one or more of them get it on Steam will we be able to play together? Thanks in advance!
EDIT: Oh I also noticed a lot less expansions on the store page here than on Steam but also read somewhere that more expansions become available over time - is that what is going on there?

Yes, you can play online with Steam players (also iOS, Android and Nintendo Switch) by using the GOG version, but you'll need to install GOG Galaxy.

Also, all Talisman content is available on GOG, but it is possible that some expansion bundles are present on Steam but not on GOG.

More Talisman content is planned by the devs, but there's no ETA.