GeneratorFabricMod VS SeedCracker

I didn't watch the tutorial you linked but for creating the basic mod structure I'd actually recommend either cloning the example mod, using GeneratorFabricMod or generating a project using the Minecraft Development plugin for IntelliJ.

Did the tutorial use GeneratorFabricMod (https://github.com/ExtraCrafTX/GeneratorFabricMod)?

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.