So I saw there is a way to modify the amount of turns it takes to traverse a wormhole, in Registry.xml as WormholeTurnCost under PathFinding (set to 1 by default).

However, this does not seem to be working properly. Setting this value to anything higher than 1 will increase the cost to traverse it, but the game will skip the extra turns if pressing the "Move All" button or ending the turn. Only by moving ships manually does the cost work as it should.

Does anyone know of a way to make this work properly?

I've been looking, and I cannot find anything about altering wormhole movement cost. Where did you see that info?

My suggestion is to look at that vaulter tech. The 100% -> 75% movement one.