Almost every router except mvebu supports WPA3 on Openwrt. There are plenty of devices that are cheaper $100.
There are a lot. Use TOH to search for one. But you can't find all in your country.
I can't trust that list, bacause the WD N750, which has problems, is also there: [OpenWrt Wiki] Table of Hardware
I don't want to waste more money, because of that is ask here.
Question, a lil bit closer: Who knows routers from WELL-KNOWN manufactors (available everywhere) they are FULLY compatible with OpenWRT 21.02.3?
Warning: 21.02 release does not work. (March 2022).
-> [OpenWrt Wiki] Western Digital My Net N750
But it's in the compatibility list!!!! WTF!!!!!!!!
"Almost every router except mvebu supports WPA3 on Openwrt"
Is WPA3 in some cases not a hardware thing, or at least a mix between hardware & software...?
Or is it a pure SW thing, so i can use a 15year old router that is OpenWRT compatible for using WPA3?
And on the other side: The WLAN module in my Acer Predator works with WPA3, the Edimax "n" WLAN stick on my old laptop not? Just a problem of Edimax drivers? Or a problem if the stick (hardware) itself?
since old, pre WPA3 hw works with WPA3, it got to be a SW thing, right ?
I think that's right... 
off-topic, no-wrt-related question: Maybe there are some manufactors, they implement it in a FPGA or something like that?
I am using Tp-Link 1043nd and dir-825b1 with WPA3. They are very old.
It depends on drivers. mvebu doesn't work because wireless drivers are not developed any more.
still a sw thing, no matter what, right ? 
It is a software feature. It is correct. But in some cases it is impossible to make a software solution without knowing how hardware works.
WPA3 itself is a software feature (depending on a recent enough host OS/ kernel, hostapd/ wpa_supplicant), but it depends on IEEE 802.11w to be available (and work). Ideally this is supported in hardware, but most mainline kernel drivers can fall back to the generic mac80211 implementation in software, if that is missing (at a considerable performance reduction). Then there are the fullmac('ish) chipset designs, which do handle large parts of the connection handling in hardware (firmware), in many of these cases the drivers can't fall back to the generic mac80211 implementation anymore, meaning the firmware either works (ath10k/ arth11k) or it doesn't (brcmfmac (802.11w not available), mwlwifi (hard crash)).
With mac80211's (software-) fallback for 802.11, even ancient 802.11g chipsets (ath5k/ b43) can do WPA3 with 802.11w, but with more CPU requirements than would be needed without it (and relying on hardware accelerated encryption) - this is generally not be an issue on the client side (fast x86 hardware, even old hardware is comparatively 'fast'), but would be for chronically underpowered WiSoC designs (aka plastic routers).
Back to the topic: The Western Digital N750 seems to have a serious (irreversible?) problem after playing with current OpenWRT release - because of that i ask my self why it is in the TOH compatbility list:
-> So i saw, Linksys had some (official) compatible devices from the "WRTxxx" series - somebody has experiences with that devices?
The Marvell devices have semi broken 802.11n wifi, and WPA3 makes them crash.
Not sure if the WPA3 issue applies to both radios, or not.
Thank you for your fast feedback!
It does, WPA3/ 802.11w crashes mwlwifi (2.4- and 5 GHz). Plus all the fun with client interoperability (esp8266/ esp32) and the drivers being abandoned for years now; avoid at all costs, this is a dead end.
I recently bought a 2nd hand Wavlink Quantum D6 for ~ 20€ that works with current release and was easy to reflash. Even branded for big electro chains they are easy to spot by their looks...
Thank you for your feedback.
(But that brand does not seem to be "mainstream" and difficult to get... my question is more related to mainstream/big brands)