Fair enough. I've taken digital logic classes etc. in college but when it comes to practical circuit design I'm mostly self-taught, having worked my way up from all through-hole components with ATmega and similar devices to dipping my toe in the SMD waters with larger TQFP parts to now doing a few designs with the RP2040. I've tried to educate myself on best practices and follow recommended layouts and things like that where possible, to try to learn to do things the "right way" - but I definitely don't have enough experience to know when the rules of thumb can be broken or not.
It's good to know that the "closeness" requirement of decoupling caps is perhaps not as important as I had believed.
The professionals use PCB-design software with physical modeling / FEA to calculate estimates to all of the important parameters of the circuit board (including how much trace lengths matter... but also board-capacitance, resistances, and resonance frequencies of the board itself, etc. etc.)
In contrast, we hobbyists deal with "rules of thumb", because none of us will spend $4000+ on professional PCB software that run these calculations for us. And furthermore, we aim very conservative because its very difficult to debug a PCB layout issue... as we hobbyists are functionally blind to all of these issues (ex: trace inductance, trace capacitance, or other issues).
I think spending a good bit of time on PDN / grounding / etc. etc. study is very worth your while.
2+ hour talk on just the issue of good "grounding" design in PCBs, but it does relate to this issue of capacitors, trace-lengths and the like. I feel like you'd benefit from this talk.
The "correct" way of thinking is exceptionally complex, far more complex than what is taught in colleges. But you have all the basic ideas thanks to the old rules of thumb. You just need to take the next step to see what the problems are.
--------
And as you'll see, traces on the same side of a board are cake. Its things like vias that actually wreck you.
It's good to know that the "closeness" requirement of decoupling caps is perhaps not as important as I had believed.