I don’t have experience of Dux Brit, but it seems to get a lot of good press.
I have played lots of Saga, it is a good game, but it certainly can feel a bit more on the heroic (if not fantastical) side of history, with some of the battle board abilities. The new version of Saga cleans up the rules nicely, it is a good game to play, fairly straight forward around the basic rules, but gives a good interesting game. The real depth comes in the battle boards (which need the custom dice). There are options for DIY dice, but having done this, I’d just buy the ones you need. The battle boards can take a bit of learning for each faction - so that you know the different abilities and when is best to use them.
For solo play the battle boards probably make it difficult. Having to manage two boards wouldn’t be easy - especially if you are new to the game.
As you already have Dux Bellorum and Lion Rampant what doesn’t work with those rules for you? I’ve played both a tiny amount, and neither really floated my boat, but it might help others with suggestions.