After a year of reflection and after having clashed with the harsh reality of consumer routers even at the highest end, I decided to come back here among you and I ordered a new Omnia (this time comfortably from Amazon). My last router was a gaming router with Wi-Fi 6 and really high performance in real life. A router paid for 500 euros. But only one update in a year, I don’t know what’s going on in it, what’s behind the software. I feel like I’m going crazy. Surely Omnia will have lower Wi-Fi performance, also because the Wi-Fi is type 5 (OpenWRT supports 5 at most), but being able to always have the latest security update, above all have the Knot Resolver (I missed it very much) with integrated DNS over TLS and DNSSEC (a marvel) the loss of performance is worth a thousand times (however not very serious).
You can change the wifi cards in Omnia. Some people do it. Search the forum, you’ll find a few tips for tested cards here.
You can also watch the Wifi 6 topic: WiFi 6 (ax) adapter . There is hope that Wifi 6 could work on Omnia (but probably not top-speed, as it will get capped by either PCIe performance or some other factors).
Thanks so much for the suggestion and for the link. I’m thinking about various solutions to improve the home network and the best seems to me to use ethernet cables and connect at least one other Omnia and cover the main home devices (Smart TV, game console, PC, laptop) and free up the Wi-Fi band for those devices that do not have an integrated ethernet port. In addition, I evaluate the use of separate Wi-Fi access points. Wi-Fi 6 doesn’t seem like the right option right now, until everything is thought of from the ground up.
You can also consider Turris MOX instead of a second Omnia. This way you get exactly the configuration that is useful to you. Or just a plain switch in case you just need more ethernet ports 
Devices used as APs or switches don’t need such high standards for security/updates/… as your router, I believe. For now that might be the best way to get the new WiFi if you really want it (or just want to utilize the device you already have).