In regards to STL speeds: That's not how space travel works. Top speed isn't a thing, and what is more important is DeltaV (Or, how long a ship can fire its engines + How powerful those engines are). A ship can have an engine that produces a small amount of acceleration but still make a trip in less time if it has a large fuel tank. I think it makes more sense to just detail what types of ships there are, and not try and jam everything into a catch all system.
The start of such a system is here: https://shattered-universe.com/doku.php ... ship_speed
We also shouldn't lump the STL and FTL discussion in together since things will get mixed up.
We should also develop basic rules for FTL, since being the fastest thing in the setting inherently also makes it the most powerful weapon in the setting. Don't tie it to gravity wells, and don't try to explain it using sci-fi works because you're going to do a bad job of it. FTL is a prime candidate for being a 'black box' technology in the setting: IE, characters know what inputs go into it, and what results come out, but the interior operation isn't something we need to describe.
IMO gunbuster's FTL setup works the best for the setting: (You can ignore the video, it doesn't really show how the FTL is used in the setting, but it does include their unnecessary explanation: https://www.youtube.com/watch?v=6qYQEEsqSJY )
Because 'space is tricky' ships can only make short FTL jumps, and require multiple FTL jumps to reach anywhere. The time between jumps is also somewhat considerable as things like passing radiation can make it difficult to jump again. So lets go with the following rules:
1: You can only jump ~1ly at a time.
2: Conditions are such that you can't always immediately initiate a jump.
3: It takes a lot of time and energy to prepare a jump.
4: The intense radiation put out by stars makes it difficult to jump into a star system.
This solves a bunch of problems. This means that travel between areas in the setting can reasonable move at the speed of plot because the time it takes to travel between systems can vary. It means that you could reasonably see an enemy fleet approaching as they make the multiple jumps towards your star system provided you've got some sensor-things way out there. It also limits the ability for people to just instantly jump-away or FTL into a planet but still leave that door open if it fits whomever's story to do such a thing. This also gives us a pretty good excuse for why supply/support ships loaded with extra fuel/energy would be accompanying a ship on a voyage between star systems if you want such a thing.