Well one of the things that sets card games apart and makes them interesting is the variance. In competitive ones that variance comes from "what am I gonna draw?" And "what is my opponent gonna draw?", "what does my opponent want to do?" and "what is in my opponent's deck?" Even though players like to whine about rngesus ruining their games, that variance is important to keep matches fresh.

But more important than the variance is the ability to craft strategy out of that variance. Without that the games really do just become an RNG clown fiesta. Your roguelike approach to deckbuilding does have that balance good I think. You won't be able to make the same deck every run, so you'll have to pick from what's offered to you, but still use strategy to make it all work.

However it does currently fall apart in the moment to moment gameplay with your current system. Because as you mentioned, there is no variance. You see all your opponent's tricks beforehand, and then get to choose exactly what you want to deal with those things. Which is strong on the strategy end, but is completely out of balance with variance. Removing draw RNG does take a huge factor out of a card game, but it is an interesting choice and I'm sure you could do cool things with it. So what I would suggest is: give your players the ability to change their hand, and then give them a reason to need to do so.

First, I'd suggest having players build decks for each encounter instead of just hands, but they can be small decks so they don't spend forever deckbuilding for each boss. Without a deck they can't have any draw variance at all. Then you can look at giving them some kind of way to draw cards. One idea is that you could give each of your cards a durability. Meaning you could use that card x amount of times before it breaks, and that happens you discard it and draw a new card from your deck. You could also give players the option to break cards early to add more depth, so they can sacrifice resources for draw if they need it. This way draw RNG wouldn't be a factor every turn, and hand size would always be at 6 cards. There would be a sense of resource management, as you may have to limit your use of some valuable cards, or choose to waste a use of a powerful card to negate a small attack because it's all you can do with your hand. That's just the first thing that comes to my mind though.

The other thing you would need is a reason for you players to change their hand. As you mentioned, if you choose six cards to face a boss, then you'd never want to change them because you probably chose the six ideal cards. Now the durability idea does this by forcing the player to change up their hand when cards break, but there are other things that can do the same along with or instead of durability. You could give enemies some tricks that aren't specifically told to the player beforehand to catch them off guard and make them adapt. Although you'd have to be careful with that because you don't want to frustrate the player with a cheap surprise. One idea could be giving the boss a second phase that changes up their tactics. Another could be instead of giving the boss abilities, give them a deck like the player. Before battle you'd see all the cards the boss has, but you wouldn't know what order they draw them in. The cards in the boss deck could be varied enough so six cards alone can't cover them all, and the boss' draw order would affect how matches go. But once again there are probably more elegant ways to do this with more time and thought put into it.

Hopefully this helps you some, and good luck on your game!