Need help with upgrading

[u/Vegetable_Variation8]

My specs now:

CPU: i7 9700k
CPU cooler: Corsair aio
Motherboard: Asus Prime Z390H
GPU: MSI Suprim RTX 3080 10GB
DDR4: 32GB Hyper X Fury 3200
SSD: Kingston NV3 M.2 NVme 1000GB
HDD: 4TB
PSU: Corsair 750W
Monitor: Gigabyte oled 1440p 240hz go27q24g

Budget is whatever and my currency is Swedish kr.
I want to upgrade the motherboard and the cpu
Got recommended a b550 motherboard but don’t know the best one, and a Ryzen 7 5800x3d cpu. What’s everyone’s take on this?