That's rediculous. A 3080 is more than enough for a less than 4k resolution. You won't get super high fps with ray tracing enabled but thats also true for a 3090. For all practical purposes a 3080 or even 3070 would be fine.
This is soooo not true! The bandwidth is definitely more than a 4k@60 monitor. It's a fact! This monitor is almost as power hungry as 4k@144 monitor.
How's 5120 x 1440 @ 100Hz 10-bit less than 4k bandwidth? You're not even taking into account that this is a 10-bit monitor.
The 3080 would be the perfect card for the OP's monitor for under 90FPS in AAA games.
I would consider RTX 3080 as the minimum.

I would get the 3090 or wait for 3080Ti.
Samsung CRG9 can run at 100Hz at 10bit or 120Hz at 10-bit, but with color subsampling (4-2-2), which not an option, if quality is mandatory.
So:
4k@60 8-bit 14.93Gbps bandwidth
4k@60 10-bit 17.92Gbps bandwidth (Do you get the feeling now?)
4k@60 12-bit 20.9Gbps bandwidth (The new ASUS Dolby Vision Monitors: The ProArt PQ22UC & ProArt PA32UCX)
4k@120 8-bit
29.86Gbps bandwidth
4k@144 8-bit
35.83Gbps bandwidth
NOW CRG9!
5120x1440@100 10-bit
26.54Gbps bandwidth (max refresh is 100Hz at 10-bit 4-4-4 color sub.)
5120x1440@120 10-bit
31.85Gbps bandwidth
5120x1440@120 8-bit
26.54Gbps bandwidth
5120x1440@144 8-bit
31.85Gbps bandwidth (OCed)
For that resolution you DO NEED at least 32GB RAM, so your GPU won't start swapping memory to your SSD at AAA games, and to avoid any stutter.
The RTX3080 is a beast, but not for such a monstrous monitor!
It would max your monitor FPS with the older titles. May not be enough for AAA games. And it
MAY NOT be good for the next few years for upcoming titles.
I doubt that even 3090 can max that monitor out.
Why don't you buy the Radeon RX 6900 XT instead?
Couple of links to comparable monitor (4k@144 RTX3080) with similar bandwidth: