To be completely honest, the Minecraft Forum was the main website to discuss mods on a few years ago, but most modders have already moved on from MCF.
You could always create a new Minecraft modding-related forum if you want to. However, there are still quite a few popular places to discuss mods on:
- Planet Minecraft. Now, the PMC modding forums aren't as active as MCF's, but it's still an option that's available. A few modders have already transitioned over to using PMC once the news about the Minecraft Forums being archived was announced.
- The Feed the Beast subreddit. Even though you said that you don't really like how Reddit is structured (I don't either, especially for "serious" discussion), it's still there and used by modders regularly to promote and discuss mods.
- The Feed the Beast forums. Basically just a forum version of the FTB subreddit.
- CurseForge. Nearly all modders use CurseForge to host their mods, but it also has a small forums section that a lot of people don't notice. It is a lot slower than MCF or PMC, but that's because everyone just uses those websites (or others, like the Discord servers that I mentioned). Hopefully, since MCF is being archived, we'll see a few more active users over on there.