Glowstone VS mammoth

Assuming you mean this Glowstone, you'll see they themselves acknowledge that "it is not ready for survival".

It's definitely not a realistic replacement for modded Minecraft in general, since that's really hundereds of games and Mineclone only aims to replace Minecraft itself. For modded, I'm not sure what the answer is. Even most third party clients/servers that are compatible with vanilla Minecraft don't support Forge mods. Stevenarella might support Forge to some extent, but I'm not sure how it works since Forge is written in Java and Stevenarella is written in Rust. I'm not aware of any third party servers that support Forge in any capacity, but Glowstone (which actually seems like a pretty good third party server by the way) does support Bukkit plugins.

Also to add to this, if you don't need full vanilla functionality and are more focus on higher player count than something slightly more experimental but more performant might be your best bet like GlowstoneMC (glowstone supports paper plugins) or Minestrom

You could. Glowstone is a Minecraft server written in Java (without any of Mojang's code) but it is still not feature complete from what I heard, so things are missing. When I tried it, it seemed pretty solid and fast though.

I forked a project [1] in 2015 to remove Gradle, and it then quickly subsumed the original project, remaining under active development to this day.

There were other reasons, but de-gradling was one of the main motivations for my fork, and among the first of the major changes I made. The project is an implementation of an API which was discontinued by the original developers, but initially was built using Maven.

After switching from Gradle (which the project switched to in 2014) back to Maven, build times significantly decreased and development became much more pleasant. I found Gradle to be like a speed bump slowing down development, and reverting back to Maven was like a breath of fresh air. Simple, straightforward, and fast. Maven may not be perfect, but it does the job well.

[1] An open source Bukkit server implementation, https://github.com/GlowstoneMC/Glowstone -> http://github.com/GlowstonePlusPlus/GlowstonePlusPlus

But maybe soon?

Yes

80 is a ton of players though, you might be able to get away with it as long as you have very powerful hardware. This might get better in the future if Mammoth "Seamless" mode gets stable.

There really isn't a networking library that is available that a new planetside server could use. I think spatialOS had a lot of hype around it back in 2018, but for various reasons its not widely used. There was a recent project that hypothetically could handle the scale of planetside 2 with good performance, but its still in the very early stages, and probably will be for a long while.

Possible? Sure. WorldQL's Mammoth should be able to accomplish this, though it's still very much in development https://github.com/WorldQL/mammoth. Seems to be far from ready for actual server use.

After a quick google, it looks like WorldQL and Mammoth may be what you want. They say (and show, actually) they've tested it with up to 1000 players over multiple servers (on a single world) and kept a solid 20 TPS.