Just realized the M109A6 entered service in 1991... wow...
I'd say give the 109A6 an RoF of 4 rpm but increase price to 90 accordingly. Then the MSTA keeps its 5 RPM, with a price of 95 (or is that unfair? It does have 3 front armor though that is seldom a factor in its usefulness I imagine) and the AuF1 with 100 price and its same RoF.
I dont want this to seem like a MSTA nerf/Paladin buff though, so is there anything the MSTA needs really? It got its unit limit sliced significantly last patch so not sure how important that is nowadays. Would the slight RPM boost be enough to encourage people to buy the M109A6?
For the M110 and 2S7, match prices and keep 10 accuracy with 2 RPM, HE7 rounds. They'll be the big hitters, but awful RoF to balance.