How do you flash a GApps package on a fully unencumbered Android 12 system?

I have this Chinese board - a Firefly ITX-3588J with a Rockchip RK3588 ARM SoC - which is not your "typical" Android device but is fully capable of running Android 12, and I currently have it loaded up and running right now. As a full general purpose computing board capable of functioning as a cute small desktop, it doesn't have any of the usual encumbrances (e.g. locked down root, locked down bootloader, etc.) that come with Android phones that would otherwise prevent installing different software: you can even install Linux directly on it, but I have had no luck in figuring out or getting answers to if it's possible to somehow modify the stock images provided to dual install it along with Android so I'm more focusing on trying to get a fully operational Android 12 system instead.

In particular, the stock barebones Android 12 system image comes with no Google Play Store or Google apps, which suggests the need to install them manually, and I've had minimal luck trying to manually fit the various .apks involved on likely due to dependency and configuration issues. And so it seems that, after much digging and headaches (given I've never done anything like this before), the best way to achieve this is to use a premade GApps package like OpenGApps or NikGApps (I got the latter because the former doesn't appear to have Android 12 support yet, only Android 11), and then _somehow_ load it onto the board's memory card.

The problem there is in that. How do you do that? Everything talks about "flashing" as though this were a commonly understood process (but can't be the same thing as a full system image flash as that overwrites all data and here we just want to add some), and after more headaches trying to plumb that secret knowledge, I find that one way to do it is to use ``adb sideload``, i.e. the Android Debug Bridge (ADB) platform tool which I have already due to other, programming-related Android use. Yet, when I try to boot the board to the Android recovery mode and then sideload the NikGApps package, I get an "E:Signature verification failed" error with the flashing failing at about 47% or so, suggesting some kind of security-related interlock is preventing me from installing the package I want.

How do you get around this? Other sites have yet to respond adequately to my inquiries, so I'm coming here now and hope a reply will be forthcoming. Someone suggested you need to use a custom recovery pack - TWRP appears to be a common one, but TWRP is not yet supporting Android 12, and they have not responded when I asked what I should use instead. So what can I use?