I'd be cool with a 15 or 20 move max on any given ship... if there was a way to jump across the map strategically.
Like the already-existing Starbase modules that do exactly this..? No-one uses them, because they require a military ring and I don't think anyone has ever built one in the entire history of Gal Civ 3, but it's quite possible to build a starbase network that triples your ship speeds.
This fact alone basically demolishes about half the arguments in favour of massive engine stacking, tbh. No-one actually needs engine stacking. If you need to get from one end of your space to the other quickly, then the game provides tool to do that outside of giving every ship 20 stellar folders. If you need to be in enemy space, well, you need to be operating at a speed where the enemy can react to you. You're no longer just asking to be able to get from the one end of the galaxy to the other, you're asking to be able to attack multiple targets in one turn from way outside the AI's awareness, and that's the thing about engine stacking which is so imbalanced right now.
I think any change that makes military bases worth building is a good change. I did actually try these guys but I was never able to figure out how they work, enemy ships never seemed to be impacted by them and my own ships never moved noticeably faster near them. So I gave up trying.
I think bringing back the stargates from the lore that were how the early wars were fought before humans strapped fusion power plants to hyperdrive and gave the tech to everyone might be worthwhile to investigate. Being able to teleport a fleet from point a to either point b or point c at a cost of 1 move as long as b or c is adjacent to a military ring would be a nifty solution.
But however these existing techs are supposed to work.... I can't figure out how to use them.
As a total aside, I still do have a use for military bases, I build them in irrelevant locations, like the outer wall of the galaxy at the back of my empire, then offer them in trade. Here, have this worthless but mega valuable thing for your best tech and all your money.