If you can make it so that world generation doesn't affect the main and nether dimension you'd go a long way towards stability when upgrading to newer versions of MC. You can keep the main dimensions and dump other dimensions when you upgrade.
Modded Minecraft
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Theoretically there is nothing that limits you from having a modded forever-world as-long as the mods do get update per release.
However every server i created based on large mod packs has died within the year. The reason is simple, those packs are super complex and so is the risks for bugs, glitches and corruptions. Its possible to maintain the world, fix corruptions, ban problem-causing items/features. But if you or other players are even a tad to ambitions your gonna spend more time fixing the mods then actual playing.
Chances are also slim those mod-packs actually do manage to update without issues. If you want to keep the world for a long time i recommend as little mods as possible. Adding mods later is super easy, removing a single one can brick your world. Even if your world has only a single mod you may hit a road block. I am still waiting for thaumcraft 6 (Beta release was in 2019, since discontinued)
There's also the fact that mods might change drastically over time. Like thermal expansion+ add-ons. Augments changed at least 3 times, there's less and pipes, etc...
Yeah this would be possible. You would need to be diligent with backups if you planned on adding mods as the playthrough progresses, as adding mods to an existing save can cause unpredictable behavior.
An easier to manage idea would be to pick a suitable mod pack and play that. I've been hosting a Valhelsia 3 server for my friends and I for a couple weeks and it has been a lot of fun. Valhelsia 3 has all the mods you mentioned aside from Ad Astra. Valhelsia 5 has all of them. There are tons of other packs you could play too. I prefer to play packs that are based on 1.16.5 and before as the performance is significantly better than anything that came after.
Well an already existing modpack would definitely help in a lot of cases, although I felt that by using a self-made modpack could help in getting just all the necessary mods, which means less issues after a major version update.
I'm not too familiar with the topic or the mods you've mentioned.
But I occasionally help people fix and setup Minecraft servers with mods for people.
Since you're interested in long term support. Try to stay on a modern version of Minecraft 1.20.5 and newer. (If you must 1.17.x is the earliest version you should entertain) Additionally sticking with Fabric based mods you'll benefit from the newer design paradigm that values lightweight design.
Additionally consider setting a world box limit and generating the entire world before you play on it.
Hmm, what makes a pre-generated world better for long-term support?
With my servers most of the crashes have been related to players trying to do stuff during worldgen and race conditions of mods trying to do stuff before the block has fully populated