The Korg Opsix is a great modern take on FM synthesis, if you're looking to buy something new that can do what the TX81Z can do. There's a digital version as well, but my gut says there are probably better digital FM synths - I haven't tried it tho.

To your two points, it's got 24-32 voices (https://www.korg.com/us/products/synthesizers/opsix/specific...), and while it's not natively multitimbral, you can hack it using key-tracking to create pseudo multitimbral sounds (https://www.youtube.com/watch?v=B8skEWKbfBs)

If you're interested in less keyboard-y, more sequencer-y FM synthesis - the Elektron Digitone has 8 voices, and I think it can do 4-part multitimbral splits (the digitone keys can definitely, assume the standalone can). I'm not a huge fan of the Elektron style of doing things, but many people are.

I love the Digitone as well. it's 4 part multimbral and the 8 voice are shared across all 4 parts.

people tend to think of the Digitone as a FM synth, (and they wouldn't be wrong) its architecture is very much like a subtractive synth but in a normal east coast subtractive synth the architecture kinda looks like this:

    VCO -> VFC -> VCA

Digitone:

    4 operators FM engine -> VFC -> VCA (then ADSRs for the filter and the amp).

the other think thats kinda interesting about the FM section of the Digitone, is it uses a form of additive synthesis to create the harmonic series for the HARM parameter. Additive synthesis is one of the oldest forms of electronic sound generation. this allows you, like the TX81z to have operators that aren't sine waves.

So in a lot of respects the Digitone is more versatile that your standard 6op sinewave FM synth.

8 voices is so little... Older FM synths could do a lot more, even if they didn't have the VCFs/VCAs. Older sound modules tended to be full-song workhorses, while the Digitone has more of a tracker philosophy. That's ok, but they could at least have done 16 voices so we can have nice chord pads with bass, melody and drums.

The Yamaha MODX has 64-voice FM polyphony, but of course not a compact module (but it could be, probably, but there is no market for it).

I do nice chords pads with bass, melody and drums on the Digitone... just use those sound locks :-)

Yeah, it’s 8 sounds at a time, so the different channels voice steal. It turns out not to be a big drawback.

And both Opsix and Wavestate are built around raspberry pi module. Kinda wild.

I'd love if company that's, well, versed in building good music hardware would just release hackable version of their synth

Whoa! I had no idea! Very cool.

The closest thing from a mainstream synth company I can think of is the Korg Minilogue xd (and a few other things that use the same engine) where they released an sdk for the digital oscillator.