my could be something as this:
1) image quality first (24MP at least, those overpriced 20MP camers like D500 are absolute non-sense)
2) build quality inc. weather sealing + features like 2 card slots
3) available lenses and it should fit into my plans for future expansion
4) ergonomics and brand ...
now example from the real world .. when I was upgrading in 2018 from D7000 I was thinking a lot about FX, to go for a D750 but finally I went with new D7200 and never regret this choice .. ofc I had to fit into budget, that's implicit ...
your situation is however very different, you're considering your first DSRL (or MILC - mirrorless) so let me break that down:
1)
all the smartest and best-looking photographers use Nikon so I assume you'd best stick with Nikon (haha) ... This is ofc a humour but I can share my opinion on it - it's ergonomics .. Nikon has great ergonomics with usually two dials and modifier buttons so you can change any settings with your camera stuck to your eye .. Competitors often have a dedicated and cool-looking dials for things like EV correction, ISO, even shutter speed - it's nonsense and annoyance, I have such a camera and I have some photographic buddies who are using other brands, for example Sony, which has imho ridiculous ergomics or Fuji, which is infamous for employing these dedicated cool-looking dials which are just cool looking and that's it ... Other option would be Canon, they have good ergonomics too but man, do you really want those white plastic lenses so everybody with camera around will giggle ... or do you wanna be like that Peter McKinnon guy

, no, I don't think so ....
2) Nikon F-mount (DSRLs) is probably the most widespread camera mount in the world .. the most of avail lenses, new and used in the world. They also have same mount on all cameras with interchangable lenses, FX and DX so you can be upgrading only body and you can use FX lenses on DX body (and it will even usually give you better quality) ... Canon have different mount for DX and FX - however Canon would be also a good choice because the amount of avail lenses is huge (ofc if you don't me what has been said above ... haha)
3) now the thing to consider is FX Vs DX and MILC Vs DSRL ... just to make a long story short ... mirrorless is probably future and dsrl is past so perhaps go for a MILC ...however Z lenses are really overpriced and there are not many around so take this in consideration - it will hit your budget .. DSRLs are relatively cheap (you can buy some used body) and there are a lot of lenses for nikon F-mount ... all and all together, I would recommend as a new camera Nikon Z50 in kit with 16-50mm and 60-250mm (it's DX so it's equivalent of 24-75mm + 75-375mm on FX, crop factor is 1.5) for
just $1200 .. it's imho the best deal on the market now .. you can additionally buy FTZ adapter and use all those nikon F-mount (dsrl) lenses .. If you want to go for a DSRL then depends on your budget - with high budget, go for some FX, with mid-low budget just go for (used) D7200 - it's probably best DSRL camera in the meaning of IQ .. only drawback that hurts a lot sometimes is that it doesn't have a flipping/tilting screen but you can live with that .. avoid overpriced 20MP nonseses like D500 or D7500 .. OR as noted above, go for some Canon cameras (you can always use black fix to overpaint those funny white lenses and don't mind if people will be giggling that you're like McKinnon

... he's not that bad guy ... only bit disco ..)
cheers, ~d