Dirty + Drag drives, or for small ships Dirty + Drive Distributors is the only thruster engineering worth doing, even on an exploration ship, with one very specialised case:
Extended periods Silent Running Clean Drives can slow down heat build up, but the work around for that is heat sinks. And when do you ever need silent running for more than a minute or two? So even then, going as fast a possible is usually best.
If you can make it fit inside your power requirements, A rated power plant with grade 1 low emissions and thermal spread makes a real difference to heat and ease of fuel scooping.