I have Tektro Gemini, mineral oil hydraulic (Entry level)
A little noisy, but work ok
Talked to the LBS the other day on upgrades
He said might just upgrade the calipers with a better one, (stay Shimano for the mineral oil)
& Might need new rotors, have to take it in to check.
Estimate was $100 .
Not sure if that's total or per wheel ??