I don't own the TH900 or TH900 Mk2, and neither do I own the Z1R. But I've tried the TH900 Mk2 several times, I've tried the Z1R once, and I own the Z5.
Short answer, nope the TH900 and Z5 don't sound similar.
The TH900 has a lot sharper upper mids / highs. The Z5 are a lot more relaxed. The TH900 also has more emphasis on the sub-bass, while the Z5's bass has more emphasis on the mid to upper-bass. The Z5's vocal range sound a lot warmer, perhaps a bit blunted and mellow, while the TH900's vocals sound natural albeit a tad distant and it can be sibilant with the 'S' notes.
The TH900 is V-shaped in sound signature, while the Z5 is a bit more wobbly in frequency response if you graphed it (not saying either is a bad thing). If you want a point of comparison, I'd say the TH900 would sound more like the XBA-A3 / A2, or the Yamaha EPH100.
Might be a bit tmi, but hope that helps.
And yes, I agree with you, the Z1R doesn't really sound like the Z5. The Z1R, for one, has a really thin mid-section that emphasises the upper mid-range. I didn't particularly enjoy the Z1R for that reason alone.