With the speed at which hardware development is going these days, you will likely be better off to upgrade to a complete new machine in 3 to 4 years, rather than upgrading the GPU. Since you have a current top of the line machine, I don't foresee any need to upgrade in 3 to 4 years unless you want to run something exotic, and even then the RTX 3090 will have enough juice to make it work

Upgrading the components is limited to what will fit in the case, and what the power requirements are. These systems are picky when it comes to memory, and there are some helpful posts on this board from users who upgraded their memory and post the full part numbers of what they exactly got.

The size matters less than the quality of the cooling solution, and Gamersnexus on youtube recently did a tear down of the Alienware RTX 3090 card and was impressed with the cooling solution used on it. He in general does not like DELL products often.

For lifespan I would say keep it in a well ventilated cool low dust place and keep an eye on your GPU and Vram temperatures. I would use MSI afterburner for it, and GPU-Z, both free programs. I would also change the default fan curves to be a bit more aggressive and tune them for optimal cooling without causing a large amount of noise.