I think largely we are aligned on what we are looking for in a platform. The private blog idea is interesting. I normally consider blogs as public, are there private blog platforms?

Sure. If nothing else, you could proxy it through an authenticated endpoint, requiring people to log on to view it. But I don't know the blogging software space very well - there are probably projects with built-in support for this. I've started looking around; I suspect the ideal platform isn't so much a blogging platform, but it's designed more around a blog design.

If you come across one, please let me know! I'll keep updating that CryptoPad document. I also started a spreadsheet, which is better suited to the data than a document table, but CryptoPad doesn't have the ability to embed assets from other documents (other than images), so I'm just doing the table manually.

On the other hand, projects die when the maintainers lose interest.

Absolutely. Good projects attract multiple maintainers; there's a bit of Darwinism there. When one project I used was archived, I offered to take over maintainership; the author didn't want to hand it over to me, so I hard forked it and worked with distributions to replace the no-longer-maintained version with mine. It's the OSS lifecycle, right? And the best thing about OSS - if the maintained loses interest, someone else can simply take over. And if no-one does, maybe it isn't worth maintaining.

I would like a platform that I know is going to stick around.

This is so important! Especially for this purpose. Getting several people to join a platform and then put content on it introduces a lot of technical inertia. That's why it's important for me to reduce the odds of the project changing their terms of use; increasing costs; moving popular, free features to the "paid" column; and other shenanigans.

On the other hand, something like Zusam, if the maintainer loses interest it will likely also die.

See, I don't believe this. It's possible the project would die, but so often have popular projects lost their maintainers, and new people step in. They fork it, or have a peaceful transition of ownership, but the project carries on. Yes, some just disappear into obscurity, but the popular ones tend to keep going, sometimes under other names. X11 to XOrg; OpenOffice to LibreOffice; OwnCloud to NextCloud; so on and so forth. And increasingly, many projects add data migration paths from other projects, especially if they're popular. Many ActivityPub servers can import Mastodon account data, for instance.

I do have reservations about HumHub, but it's the first platform I've seen that even comes close to being a familiar feel for users.

It does look pretty close to ideal for what we've been discussing; I need to install it and try it out, because so far all other options have failed in some way. There's another forest of options in the blogging style, so I'm still optimistic, but I may try HumHub anyway.

I'm considering the other idea of using Dokuwiki as well, which I guess comes in as being more similar to your blogging idea.

Yeah, that was an interesting avenue; I suspect the user client experience will be where that fails for me. It can't require any technical expertise.