To start with HL does work with 3.5 you just need to get the right packs. The 3.5 stuff is labeled d20 BTW.
As for RW it can be overwhelming, the blank page problem, for new users. If you've watched the various videos Josh and I put out the best advice I've got for you is to download the free realms and look those over to see how those are built. If you are feeling particularly ambitious you could try importing this realm which is quite large and is being built by a group of community members and should give you quite a good look at the entire gamut of RW.
https://drive.google.com/drive/folde...I6V5FZW5HThnsQ