Terra VS SeedCracker

But if you do find a place that sells pre generated worlds with the use of Terra for sale then I can happily announce that in ther Licensing agreement it is allowed to be doing Commercial usage of the plugin so you don't have to worry about breaking any TOS. source

Hey, it is a custom server which is running on Purpur, a fork of paper that further improves features and performance. The custom terrain plugin I am using is Terra (https://github.com/PolyhedralDev/Terra).

Terra uses the datapack route.

Terra is an extremely customisable terrain engine, which allows you to make your own terrain generators. It's very in depth, but does have a fairly large learning curve. I highly recommend you join the Discord and check out the GitHub for examples, however.

World Generator: Terra

If you don't respect this, you can use https://github.com/KaptainWutax/SeedCracker

Custom code, usually. KaptainWutax made a program a few years ago which can crack seeds just by scanning the world. When seed cracking, though, you are looking for certain features which can rule out many many seeds very easily. For example, finding specific dungeon coordinates and mossy cobble patterns can narrow it down dramatically. For this process, you need not scan that much since it's some simple maths applied to not that many blocks and not checking very many times.

Im not an expert but cords are needed or at least if you want to solve it in any time. you say you play on that SMP so maybe use seedcrackermod but KaptainWutax. It is a fabric mod that searches the world for structures like shipwrecks and once you find 5 of them it can crack the seed within minutes or even less (depends on your gpu)

Otherwise, I found a mod that will calculate a server's seed but you have to actually be on the server; most of the actual tools seem to be private, most likely due to security concerns as stated above.