Going off of the current forum layout, I would remove/archive the following sections first, just to get a few things out of the way on the forum, and start making Discord the place for these topics:
Admin Complaints
Player Complaints
Appeal Ban
Become an Admin
Each server's section
Map Suggestions/Removals
Let's start there. They're not really used these days anyway, so we might as well move the center for these topics there, and make it known in the servers via adverts or whatever, that Discord is the place to report issues, make suggestions, and apply for admin. Just get the server-specific topics for suggestions/support moved over to Discord. The off topic sections I guess don't really matter one way or another if they're on the forum, Discord, or both, but I'd like to see all the stuff that relates to the servers in some way moved onto Discord. IMO let's start there.
Details on what to do with the forum sections listed above can be figured out with time, whether they stay read-only or get hidden or whatever. For the time being it doesn't really matter if they're still visible on the forum and just read-only or whatever, but let's get these discussions at least established in Discord and go from there. Maybe keep them visible and just change the forum description to "To appeal a ban, please visit our Discord" or something along those lines. For now that's all just extra details we can get into later if we enable these topics in Discord to start with.
I'd probably just use the website/forum at this point more as a dumping ground for information, or at least work towards doing that as things get moved to Discord. Lists of rules for various games/servers, list of servers, our history, staff, donate, and other important information, like you said. Whether that's one page, two pages, twenty pages, doesn't really matter as long as it's easy to navigate. If we use the forum just to post information, I would probably even remove dates being displayed on the posts just so we're not in year 2027 and still reading stuff from 2021... Simply removing the dates on information like that would avoid people thinking the information may not apply anymore or like things never get updated around here.
More on Discord...
In terms of Discord cleanup/layout/channels, I guess I'd say make it more game server focused. Right now, TTT is our bread and butter, so make it primarily focused on that server, and as additional servers are added, the layout can always change slightly as needed so we can adapt. I'll throw a rough layout suggestion out there so I'm not just saying "remove this, remove that". I'm not super discord savvy and I'm sure I'll leave some things out, and I'm not counting voice channels. Add a few channels, remove a few, whatever, just for a general layout idea though, here's something.
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COMMUNITY
# announcements
# donate
SERVERS
# server-talk
# server-rules
# list-of-servers
# become-an-admin
# map-suggestions
# complaints
# appeal-ban
# suggestions
GENERAL
# introduce-yourself
# general
# random
# gaming
# moves-and-tv
# food
# sports
# news-and-politics
GAMES
# some-game
# another-game
# something-else
# etc
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Look at our Discord now, there's a lot of quiet channels we can condense/remove for the time being. I'm thinking maybe if we have fewer channels, there's less stuff to scroll through and it's more straightforward. No one's posted in the Insurgency channel in 2 months, remove it. Yes, I know we have a server for it, but with our current state, we don't need a channel for every single thing, keep it simple and don't keep dead channels. And if you look, everything in that channel is basically off topic and could just as well have been posted in the general chat. Obviously if the general chat starts having a huge amount of Insurgency talk, then it's probably time to make a dedicated channel for just Insurgency, but that's not the case. I'd apply that logic to just about any channel.
Have a list of servers available to people know what we have, and have information laid out in a simple way that makes some kind of sense, but I think a bunch of active channels is going to look better than a bunch of quiet ones. That image is easier to maintain with fewer channels and not keeping the quiet ones around.
Also whether they're regular text channels or forum channels, or whatever else Discord has, doesn't really matter as long as discussions can be had and info can be easily found.
Just some thoughts to consider. As I said, I suggest making Discord more game server focused, as I think aside from simplifying everything, a small number of servers (and playing on them) need to be the main focus.