>>NJM2069 so that the entire originality of the Poly-800 would also be retained.
NJM2069 and MSM5232 both have influence in character of poly-800. It is better to do the emulation of 2069 in a form of standalone software that runs on PC, no need for a hardware platform here.
A good emulation of M5253 which is an ASIC chip is tricky and maybe impossible, because inside the chip all oscillators/functions are working in parallel while STM32 code runs sequentially. Also 5253 is not 100% digital, it seems there are some analog VCAs, EGs inside this chip. I'm not sure even there is 8 DACs on die for building waveforms (regarding the size and cost of the chip) maybe oki/korg used only SQR and Mixed SQRs in analog domain for building saws...
The curve of the AMP envelopes, the color of VCA and also the phase management of oscillators are the points that must be considered.