Firmware for HL-WR865 with MT7620A

I just got hold of a router with the following firmware on it:

Hostname OpenWrt
Model HL-WR865
Architecture MediaTek MT7620A ver:2 eco:6
Firmware Version OpenWrt 19.07-SNAPSHOT r10272-e0f4688 / LuCI openwrt-19.07 branch (git-19.207.38473-c0057ae)
Kernel Version 4.14.133

I would like to upgrade this to the latest firmware but can't find anything after an hour search. If there is no upgraded firmware for it, then I would like to load regular Openwrt into it in case it has a trojan or anything hiding in the shipped firmware.

What you're looking at is probably a modified vendor firmware loosely based on OpenWrt, either directly or indirectly via Mediatek's SDK - it's not OpenWrt™, nor currently supported by OpenWrt. If you want to see your device supported (assuming it has more than 4 MB flash and more than 32 MB RAM), you'll have to port OpenWrt to your device. While most Mediatek devices are relatively similar, you'll still have to do the full porting work, starting from investigating what the vendor has done with the fork of OpenWrt (sources/ GPL tarballs would be beneficial for this), before trying to apply what you've learned from that to OpenWrt (master) - there are no generic images, nor shortcuts.

considering there's only 172kb available on filesystem /overlay, it might be a 4mb flash device.

I know it has 16M flash and 128M memory. I tried installing Samba4 and watched as the memory filled to capacity. So it has flash available, unfortunately only about 10M left to work with. Still I'd like to maybe get a build that is more generic without all the cell modem support that the vendor put on it.