But......most of the problems have a solution. Perhaps it is not a perfect solution because it adds a strange movement to the ships but.......now they will target the enemy directly with their main weapons and they will be units much more useful.
In addition, you will not note it but the Mandator IV has 24 anti fighter new turrets and these weapons run much better now.
personally I love the Onager as it was a perfect specalised ship that fits into the empire theme, as it was designed for one thing sieging planets and it fits that roll the best, not these lazy up scaled isd 1 from the latest film which they just stuck a cannon under it and said "hay this thing can blow up planets now"
does your mods in STEAM WORKSHOP already have First Order Faction in campaign?