There are other M.2 type graphics cards. For example, I ran across the following card several months ago;
https://www.asrockind.com/en-gb/M2-VGA-IPC
Another user here also posted a link about it;
https://forums.sifive.com/t/anyone-receive-their-unmatched/4421/2
As noted elsewhere, you have to be careful with M.2 cards. The Unmatched uses M Key but these M.2 video cards may not be work in a M key only slot. Over all, the M.2 standard is quite complex and has led to many incompatibilities.