Four things to get right first
server.properties is one setting per line, no quotes
Spaces around the = are kept as part of the value. That is why a motd with a trailing space behaves oddly.
More RAM is not always better
Past a point a bigger heap makes pauses longer, not shorter. The flags matter as much as the number.
Plugins and mods are different things
Bukkit and Paper load plugins; Fabric and NeoForge load mods. A plugin will not run on a mod loader and the other way round.
Back up the world before every change
A bad config can be undone. A corrupted region file after a crash usually cannot.
Tools that do the work
Reference, straight from the game files
Read out of the game with Mojang’s own data generator, so it matches the version exactly.