Does anyone have recommendation for relatively cheap (could be used) and small (5-6 Ethernet ports since I'm limited with space) router that have at least 1x 2.5G LAN, 1x 10G LAN and 1x 10G WAN and support OpenWRT?
Wi-Fi is optional (would be disabled anyway)
Oh and I want to use 2.5G since I already have built-in 2.5G network in my PC, but if there is really nothing good that have 2.5G support, I could go with something with 2x 10G LAN and buy 10G network card to my PC
Serve The Home forums and YouTube is dedicated to these devices.
Protectli, CWWK, Topton are the manufacturers. Protectli is actually a rebrander of the Chinese products for western audiences. CWWK designs the actual motherboards. You can order from whichever vender you like, although Topton seems the least reliable as far as English support and firmware.
If you could relinquish to the 10G LAN port in some way may be the mikrotik rb5009 fits your needs. It has 1x 10G sfp+, 1x 2,5G and the rest are 1G Ethernet. Also its support by openwrt but I would give RouterOS at least a try.
5-6 ports is not "small". "Small" is 2-4. Six-port 2.5G units do exist, but they are relatively expensive compared to the actual small (2-4 ports) units. You have to trawl eBay or bite the bullet on AliExpress...