As for the front-end I think MIT or even GPL type of licenses make more sense, since everyone can just view that code anyway.
I would consider what kind of licensing you want to do, I would also consider goals in doing this. One huge benefit of an open source SaaS offering is the freedom to take control of what you're getting out of it in-house, or going straight to the vendor.
Will you be offering migration to and from your SaaS service? These are all key things you need to put front and center. Definitely consider what kind of license you want to use, very important.
The way something is licensed makes a big difference.
.NET and Visual Studio Code are both MIT licensed, which is why any time someone cries about "its microsoft they're just gonna take it proprietary" I point out that the MIT is the least restrictive license you could use aside from just public domain, and it makes their point moot.
As for building a community, definitely make a Discord and a forum, places people can come together and get help publicly, figure out how to get a stackoverflow space for it as well.