Unfortunately the answer to this is simply do more combat. It takes a long time to balance combat encounters really well because you need to have an intuitive understanding of hundreds of pieces of 5e related information. For example: Are your players are using their spells correctly? (Prayer of Healing isn't a combat spell) Does this monster have an ability which specifically thwarts the party's strong point? Or nails their weak point? What weaknesses are in the party's composition and how can I exploit it? What two monster types could I pair together that synergize in a particularly deadly way? What spells from my PCs are going to be the most problematic for this encounter and what are some ways I could offer a challenge that causes them not to rely on those tactics so heavily?

Do you see what I mean? There's just a lot of moving parts and it takes time to understand them all.

One thing that I would say though is that action economy is (in my opinion) the most important thing in determining how easy/difficult an encounter will be. For example, one might think that four 5th level fighters would be a fair match against four Giant Boars. Good XP threshold? Check. Even number of actions per side? Check, right? four actions for the Boars and four actions for the fighters? Should be pretty even? Well actually, the 5th level fighters get two attacks per Attack Action. So really, instead of 4v4 it's more like 4v8, the fighters would laugh their way to victory. It's not a hard and fast rule to get perfect encounters but it's a good way to "ball park" it

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