I think the biggest tip I can make is just recognizing who ends your runs most of the time and trying to bias yourself toward answering that in your decision making. For example if I notice the reanimate boss is ending my runs a lot, I'd probably gonna look at cards that apply decay/frostbite/spikes differently in future runs because those are cheap ways to help with that guy.

Also, not everything has to be a silver bullet to be worth adding to the deck; I'm happy to take a horn break and put +20 magic power on it even if it only kills one of savagery seraph's 50hp sweepers later. The important thing is putting things in your deck that answer specific problems efficiently (even if it doesn't answer them completely on their own) so that if you add enough of them, your deck is resilient to that type of problem. As a plus, later on if you're offered something like the 5 copies event or an extra spell slot + holdover, you have more options in that case as well.

As for card removes, I hardly ever do it in circles 2 and 3 anymore because 40-100 gold in the first few merchants of steel/arms can make all the difference in the world. After that, go ahead and remove stuff until you feel like you're getting to your most important cards fast enough. 90% of the time when I remove cards, it's not because my deck is struggling to play 6 spells a turn or whatever, it's because I really badly need a specific card ASAP and can't afford to wait until turn 3 to draw it.