I think Sir Lancelot and Scarlett are both good choices. Taking into account your red roster, you have Boldtusk on tier 4. Both Sir and BT are medium, but BT’s special covers the entire team and BT is much more resistant than Sir, so you would be overwriting the attack effect and Sir’s special only covers nearby allies. Furthermore, BT could become a beast with emblems, and I don’t think Sir even at +20 will be as resistant as BT.
I have kind of the same “problem” because I received Sir in a 10x Avalon pull, but I already had BT 4/70 +7, and Wilbur also full. It would be cool if their attacks boost stacked, but this is not the case. I will maybe level up Sir one day, but I have to focus on other colors, and for now he is a very expensive toy.
To resume. If you have already BT, go Scarlett (yeah a lot of complain about how squishy she is) but paired with Wilbur you will increase her survivability, and her attack is nothing to ignore