Odd, I've not suffered noticeably from the loss of frontier settlements, even for example one that was the object of a Senate mission. Perhaps completing high % of Senate missions makes the loss of some minor place "forgiveable".

The main disapproval factor I've seen playing Roman faction, is getting over the 35+ settlement hump, where they suspect you of harbouring plans for world Domination

There again perhaps building a variety of suitable temples, and having influential governors who get the Senate jobs, keeps them more friendly, for longer.

Cleaning out the whole of Northern Europe because the barbs attack you, and holding onto all those provinces, whilst not interfering sufficiently with Scipii & Brutii expansion, would seem to be a recipe for Senate troubles, without the central resources to wage Civil war effectively.

What about Trade Rights? Freely offer, or be selective, if so why?