I also have that same case with my 3090 vertically mounted, however, it's on a waterblock instead of an air/hybrid cooler, so I'm not sure how the spacing and airflow is for non-waterblocks with that 5000d case.
 
The riser cable I use is a LINKUP PCIe 4.0 Riser Cable 25cm (link: https://www.amazon.com/gp/product/B08YZ74DT9/ref=ppx_yo_dt_b_asin_title_o00_s01?ie=UTF8&psc=1), which apparently is out of stock currently.
No drilling or case modification required for the 5000d, just obtain and screw the correct standoffs on the floor directly above the PSU, the case already has them pre-drilled.
Although this riser worked fine on my games while at PCIe 4.0 link speeds, I have mixed feelings about this riser cable in general:


1) It's a little flimsy.
Mine never actually broke, but I feel like it was bound to happen after holding the weight of a heavy GPU while sitting on just two standoffs, which made the riser PCB bend slightly.
Fortunately, one reviewer 3D-printed a mounting plate for this riser and shared his files on thingverse (https://www.thingiverse.com/thing:4829462).
So I used those files to get a mounting plate 3D-printed by a third party for like $30, which in the end turned out ok for me.


2) I got a bunch of PCIe errors with this riser.
It wasn't glitching out my games or causing noticeably lower performance, but I was able to see these errors in windows event logs along with hwinfo. After temporarily switching back to a horizontal mount (without a riser cable), the PCIe errors went away.
As of right now, while back to a vertical mount, I think I got those PCIe errors to go away by reinstalling the riser by slightly adjusting the position, which also helps improve the GPU sag by a little bit.
The riser board has multiple screwhole positions that you can slide giving a little flexibility as to where the board is mounting exactly.
The currently error-free way I have this riser setup on my 5000d is with the riser board slid as rightmost as possible (in the direction of the front fans), while still aligning inside the screwholes in the chassis.
 
I am not sure if I would recommend this riser+cable after dealing with its flimsiness and the PCIe errors it generates.
But it is a working and error-free riser cable as of now, that is also PCIe 4.0 compatible based on the link speed I'm seeing in hwinfo.

I would wonder how the airflow would work with the 3080Ti having a large cooler. I have done vertical mounts but even with a smaller cooler noticed a heat increase to the card.