Yea Master Tact is definitely one of the best two drops I think, useful all around. I have 6 non-Viking cards, 4 relics and two buff cards, but during my run I've never had an issue with Runecaster, I can always play those cards first.

I thought about Valka's Captain a couple of times, but I don't like that you can't play it if you don't want to overwrite your relic, especially Enduring Shield.