Is it that spectacular that I should I consider selling my 1266 Phi TC and Susvara Unveiled to fund buying one?
Or is it just different and very much dependent on personal taste?
Very hard to give you an answer for that.
It just depends.
The 1266TC is the bass king, and if you crave its bass, the Immanis will not replace it. If you are using the new pads (no seal break) or are ok with losing that subwoofer bass, you can sell the TC.
The Unveiled is a super specialist when it comes to midrange / vocals / intimacy etc. It can truly touch your heart like the Immanis can never do. I think it is an excellent companion to the Immanis.
The Immanis is faster and more detailed than those two and has a different tonality, rather U-shaped. Mids are recessed, it won't sound truly intimate like the Unveiled, its bass is very good, but can't reach the 1266TC's bass.
So: Do you want a TOTL, estat like experience when it comes to technicalities (and with a fantastic bass, which estats can't deliver), or are you super happy with the tonality of the ultra bass slamming TC and the heart-touching mids of the Unveiled?
If you think that you won't miss the mids of the Unveiled: Sell it.
If you think that you won't miss the bass of the 1266TC: Sell it.
The Immanis is more of an allrounder in terms of tonality, and a pure master when it comes to technicalities. In terms of objective technicalities, it wins, hands down. In terms of subjective tonality, the TC and the Unveiled have both one single feature (bass / mids) where they outshine the Immanis.
You may be able to substitute the Unveiled for the OG Susvara or even a completely different headphone, like a ZMF Aeolus (sounds ridiculous, but it has the same tuning in the midrange as the Unveiled, extremely rich and full mids, but of course it's way behind in technicalities). You won't be able to substitute the bass of the 1266TC though.
Hope that helps a bit. You can always tune the Immanis more to your liking with the chain (different tubes). Immanis is extremely good with the Envy and gets even better if you have found the perfect tube pairs for you taste with it.
Edit: I see that you already have some ZMF's, so you probably won't miss the midrange of the Unveiled that much, i don't know if you would miss the 1266TC's bass, only you can decide that.
Do not keep the Unveiled just for its technicalities (details, resolution, separation, speed, etc) the Immanis stomps it in this regard. Same with the TC, if you only have any of those because of their technical abilities, sell them and try to get an Immanis / Magna. The Magna is less expensive and also stomps those two in terms of technicalities. It has a slightly weaker bass than the Immanis (still very good, better than OG Susvara) and a very OG-Susvara like tonality (less recessed mids, a more normal, less speaker-like presentation).