Why do you feel the need for additional/improved missionaries?
Let's compare the situation with Vanilla BTS:
There are only four major religions of which one comes very late into the game and is spread automatically in all of your cities. In BTS, there are seven. So, there will always be enough diplomatic partners for you, no matter which religion you choose (with the exception of Islam ofc).
The income of Holy Shrines is limited, so you can't max that out with missionaries. Additionally, it's impossible to control the catholic Holy Shrine (which is not a good game mechanic imo).
The automatic spread of religions is massively increased compared to BTS. So, while roleplaying (which is a big factor in RFC) you can pretend Irish monks have converted your pagan citizens when Catholicism is auto-spread.
The automatic spread of religions is getting an overhaul atm (if I'm correct) which will probably further improve the issue.
In conclusion, I don't think there have to be additional missionary mechanics. Imo let's just wait for and test the upcoming province-based religion spread.