YAGNI, DRY-ish (repeating 2-3 times is Good; now there's data on how best to abstract it), SOLID, etc.
Recently it's "Slow is smooth, smooth is fast" or "slow is fast". https://ell.stackexchange.com/a/132146 Too often we cut corners in an effort to get something out the door while avoiding changing the things that cost us more.
Lastly, it is _never_ too late to change a technical decision. It might take more planning and a longer timeframe to migrate, but it's worth doing.
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