TL;DR: Likely a certification issue. I have no solution. Hopefully AZERTY Astros arrives soon.
I also think it's a certification issue. I certainly think it was, when I had similar issues with the somewhat similar app BankID, which is used in Sweden, for identification and payments (sometimes as the only way). It too supports Android 6 and newer, if the device has Google Play Services or Huawei Mobile Services, and a usable rear camera (one of few things I find the Gemini add-on camera useful for, is QR codes and similar).
The original Gemini Android firmware wasn't Google certified at launch. My unit showed the S/N 01234567890ABCDEF, which clearly wasn't true. BankID would install but didn't work. A later Gemini Android firmware was Google certified, showed the true serial number and BankID worked. Then came Android 8.1.1 for the Gemini. It no longer showed its certification status, but as the S/N again said 0123... I'm quite sure it wasn't, and BankID stopped working, just briefly showing its logo before exiting to the home screen with no message or anything.
When I, out of curiosity, read the itsme FAQ, it said "to guarantee your security, itsme doesn't work on jailbroken or rooted devices", so it (and likely BankID too) tries to see if the device is stock. My Gemini always was, but without Google (or Huawei) certification, that test likely fails.
So yes, I think it's a Google certification thing. Whether Planet screwed up, were screwed over or Google raised some bar out of their reach, I don't know. By choice or not, they don't seem to work much on older devices. Even the Cosmo patch level is January 2021. After updating BankID on my Gemini yesterday, it did start, but as I didn't update its certificate, I don't know if it would work.
The BankID FAQ suggested making sure the Android System WebView app is up to date. If itsme also uses a WebView, there could be some overlap, but likely you'll have to wait for the Astro, lest you can go back to Android 7, which probably isn't reasonable. Update #67 from last tuesday (UTC+2) at least said production had begun on Astros with French keyboards. Since I know of no particular Astro version for Belgium, I guess you're waiting for such a device, so I hope you won't have to wait that much longer.