Knowing your constraints perfectly is impossible, and takes more and more effort the closer you get.

And frankly, they change on a regular basis. By the time you figure them out that perfectly, they're invalidated. A new feature is suddenly needed by the customer, a bottleneck that wasn't there before appears now, scale changes, consumer preferences change, new hardware and software come out, everything changes.

IMO, the best engineering is 80/20. Get 80% of the result you want for relatively little effort and complexity, and don't sweat the remaining 20%. Embrace the imperfection. Get it done in the simplest, most obvious way - even if you know it has limitations, edge cases, and some things missing.

Knowing how to get that 80% is extremely hard (and how to do it without causing lots of problems down the road), but I think senior engineers get pretty good at it. And if you absolutely need to, get another 80% of that remaining 20%.