Nerojabes
I want to get the new 3090ti and currently wondering, since I saw a couple of posts on how massive they are, if they will still be able to be used on a vertical mount on a Corsair 5000D. If so, any suggestions on which ones to buy?

How many spaces does your mount support ?  Distance to the side of the case ?
 
3.5 slot cooler .... probably needs a custom solution & creative mounting inside any case

im currently using the phanteks vertical mount in 5000D case, which was made for 2-slot but can easily fit a 3-slot card with the 3rd slot just hanging off the side of the mount. however, i don't recommend it because you have to hack off part of the case to access the DP/HDMI outputs.
 
you are better off using the cooler master v2 vertical mount, since it allows you to mount the gpu recessed further into the case. that way you just slide the cable through the back of the case without cutting it up. it is also made for only 2-slot but it can easily fit 3-slots same as the phanteks one. the 5000D case is also deep enough to accommodate the recessed position of this massive gpu with plenty of room between it and the front of the case, even with a front mounted radiator
 
the only thing i'm not sure about is the clearance between the gpu and motherboard. i used an older version of the cooler master vertical mount and my gpu backplate was physically hitting against the heatsink fins on trident z memory. the phanteks one at least gives a lot of room to clear tall ram modules.