The 3090 being 'futureproof' is really questionable.
I mean, it's 24GBs of Vram - whenever that amount of ram becomes relevant, this gpu will be slower than newer, faster models with less Vram, and its resale value likely won't be as good either - except for those looking for a Quadro alternative, I guess.
Depends on how many hours a day you will run them, as well as energy costs in your area, amidst the fact that value keeps changing... the latter would be too hard to figure out though.
https://www.whattomine.com/gpus
As an example[a bit oversimplified though]: Let's say you bought the 3090 at 2000USD, and you mine with it all day long.
You will break even around 336 days, or 11 months, and the money you make after that will be profit.
Instead of mining all day long, let's say you only do 12hrs a day instead.
You will break even around 667 days, or 1 year and 10 months.
How about 8hrs a day...
About a 1020 days, or 2 years and 10 months.