Thank you. I gave up on using prime, it's not a good notation anyway. For the record I'll leave here the tests I did.
I wanted alternatives 6, 7, 11, 12 but could not do it in context. Alternatives 15, 21 are almost there, but the prime is clearly smaller than the ones in 13, 19, and too small imo.
plain tex:
$$1T\prime + 2T^\prime + 3T\prime_{rs} + 4T_{rs}\prime + 5T^{{}^\prime}_{rs} + 6T_{rs}^\prime + 7T^\prime_{rs}$$
$$8T' + 9T^{'} + 10T^{'}_{rs} + 11T'_{rs} + 12T_{rs}' $$
context:
\starttext
\startformula
13T\prime + 14T^\prime + 15T\prime_{rs} + 16T^\prime_{rs} + 17T_{rs}\prime + 18T_{rs}^\prime
\stopformula
\startformula
19T' + 20T^' + 21T'_{rs} + 22T^'_{rs} + 23T_{rs}' + 24T_{rs}^'
\stopformula
\stoptext
YT