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.
https://mikrotik.com/product/rb5009ug_s_in
https://openwrt.org/toh/mikrotik/rb5009ug_s_in
Regarding the 10G LAN depending on your needs its may be better of in a DMZ anyway?
Maybe I won't even need 10 GB WAN so this could fit my needs, thanks
Protectcli Vault 4!
Probably an SFF PC off ebay for $60 or so + a 10G NIC. Could run Opnsense, or OpenWRT if you wanted to.
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...
5-6 is small for me (especially where there are routers with 24 ports or so)
Plenty of such on Taobao. I guess you can find them on aliexpress
I would look for them
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.