I'm with new_man on the dill and carrots. My charming daughter has decided that carrots sauted in EV olive oil, dill and cumin are the best thing in the world. The cumin comes from the Apicius recipe (and so does the olive oil) and dill is well known in period. Adding the pair is "plausibly period".)

If you pre-roast the garlic in too much oil, and save the oil, you can be very very very happy. :-) Too much oil might also resolve the dryness issue.

I have no period documentation for it, but plausiby period - my best pea soup is made not with onions, but with leeks. It is much better, by far. If you want some of the flavor of ham in the soup, you can use one of the Liquid Smoke variants. I have found they taste poisonous in pea soups unless used in microscopic quantities - aim to go very lightly and you should be fine. Thyme in the pea soup helps as well.