so when you look for NPCs, you don't worry about level appropriate stats or any of that, but focus on the personality and background? I'm assuming you'll then stat up as needed? Just trying to make sure I understand
Quite right. If it is going to be an npc that will need stats, I'll throw some stats together. Presumably this means they'll have a class, so basic understanding of class features/abilities are needed and probably would require a quick look up [if, say, on the fly I had to make an npc combat-ready].
I mean, I know, as DM, if this is someone more/less/comparably powerful to the party, if they are a classed individual, just by virtue of who they are.
But, yes, the background personality is more interesting to me as a DM and, I find, makes for more interesting interactions/moments in game than having a page of stats for an npc.
To use your example above: I want to throw in a dragonborn fighter for...what? Is this just some guy they run into at a tavern who wants to join their group? [lowlevel]. Is this a captain of the town guard who's responding to some disturbance the PCs caused? [mid-to-high level] IS this the uber-BBEG-mastermind behind all of the party's woes? [super-high level] Your proposal permits any of these, yes. But I don't really need the minutia of "Is he 4th or 3rd level?", or "13th instead of 12th, he gets this/that." And, it seems, this the same [personality-wise] creature whether he's 3rd or 13th? That doesn't seem like it should be the case.
Maybe, just off the top of my head here, if you are going to present an NPC across 20 levels of crunch...maybe provide, say, 3 levels of flavor?
An "Encountered as low level: he's like this. Encountered in middling levels: he's like this. Encountered at high levels: he's like this."
Make him seem like a growing, breathing character. We seem to often forget the "C" of "NPC". That way, also, if they are the type of character that can be encountered more than once over a PC's career/lifetime, they aren't just the same cookie-cutter guy they met 10 levels ago.
And by making the categories, simply, "low/medium/high" level, instead of assigning numbers, that leaves things fluid/flexible enough for groups that say "low level stops at 3rd" while other groups cna say "low level" is anything under 10, or the "9th level is end-of-middling/start-of-high "name" level" crowd and the "you're not high level until 12+" crowd can use whichever personality/background works for their definition of those levels.
Just an idea. But yeah, I personally, generally, don't need/use 20 levels of stats for NPCs.