How can I install NikGApps on a fully unlocked Android 12 system without being refused by signature verification?

I have an Android 12 installation loaded onto an ARM SoC "super" SBC (Firefly ITX-3588J) and want to try and get Google Play working on it, as the stock A12 image does not come with it. Because this is a full computer system and not a smartphone, I both have full access to the memory, bootloader, root, etc. yet also, pretty much all instructions I've found online for installing things like that seem to presume a smartphone with varying degrees of restriction. My best approach so far has been to get the package "NikGApps" (), instead of Open GApps, because the latter does not currently support Android 12, but the former does. However, I have no idea how to load the package into the system. Looking it up, most people on smartphones seem to do this using something called TWRP, but TWRP apparently _does not support Android 12_. Yet obviously the author of that package somehow intended for it to be installed on Android 12, but the instructions they provide for doing that are _extremely_ terse and simply say "flash" it to the board with no details of _how_ you do that. Further digging _seems_ to suggest the answer is to use `adb sideload after booting to the stock (i.e. not TWRP) recovery mode, however the problem here is that when I do that I get an E:Signature verification failed` error and the system refuses to complete the installation (at somewhere between 40% and 50%, I believe 47%). Presumably, and from what I can gather online, this is a problem relating to the fact it is not some officially "blessed" package for the Android system (i.e. it has to be signed and OKed by some "trustworthy" source). And so my question is, is there some way around that? How can I install this package successfully and correctly?