Anyone know if it's possible to flash better BIOS on one of those? A lot of basic games hang, I suspect it's because they depend on something in original PC BIOS that is not emulated well enough?
Probably possible somehow. Apparently it runs an open source BIOS[1] that they kind of stole. Despite this, the author of said BIOS added support for Book8088, so it should be possible to flash one of those images onto it[2].
I'd actually guess the mods in this article are more likely to fix broken games, though, especially if they're shipping with 8088 and CGA clones rather than the real thing. That said, I don't actually have any experience with the 5150 or its clones directly outside of emulation, so I can only really postulate. (Retro-computing has gotten quite expensive, so I have prioritized a lot of other machines for now, otherwise I'd really like to.)
[1]: https://forum.vcfed.org/index.php?threads/chinese-8088-based...