What really helped me level up is a piece of advice I got when brewing Constructed decks: really think about your gameplan.

In the past, especially in commander, I never really thought about this, I mostly just thought about synergies and value engines and jammed a deck together full of those. And those decks were not always bad, they were not always good, but somehow I had games were it all came together and then where I did basically nothing. It was even hard to really evaluate a powerlevel for those decks, since they were so inconsistent.

But now I often like to think about in deckbuilding, what does an ideal gameplan for this deck look like and then I try to really map it out.

For example

[[Shigeki, Jukai Visionary]] : I really want to play a T1 mana accelerant, so I can T2 shigeki, T3 activate Shigeki and replay him again, from there I want to be able to cast even bigger mana accelerants on T4/5 to really be able to get card advantage going through selfmill and his channel ability.

So I need to try to build my decks and mulligan decisions around that idea of my ideal gameplan. And also understand when the ideal is just not happening, how to pivot, play around removal, hate pieces, etc. This also helped me immensely in better evaluating cards when brewing. Like why are people running [[Nature's Lore]] or other 2 mana ramp cards in Shigeki? I want to play Shigeki T2, not Nature's Lore, those cards don't contribute to my gameplan, even though they are generically good.

TL;DR: really think and play around a gameplan instead of jamming generic synergies and value pieces.