I'm still not entirely clear on what you're proposing. But a few points based on what you've said:
If you're going from x.x.2 (with two front Atmos) to x.x.4. The two additional Atmos speakers, whether they are up-firing position or on the wall need to be assigned as either "surround" or "surround back" and need to be positioned on-top (for up-firers) or above on the wall whichever they are, i.e. the height channel for either surrounds or surround backs.
You cannot have surround backs without surrounds. It is generally advised with a sofa against wall to position surrounds roughly where you already have. And surround backs, in general, don't work because you are so close to them. Some people do it. There are some, for example who have speakers on the floor behind the sofa firing up!
The T-series speakers do not "match the R-series". I get what you're saying, they should better tonally match than what you have as they have a Uni-Q driver.
General advice would say to stick to 5.x.4 and if you want to go with KEF, then stick T101 or T301 at MLP ear level on the walls to the side of the sofa (like where your surrounds are now). And put the "surround heights" on the wall at the top above the T-series.
I'd be concerned that the sofa seating would be so close to those heights that they may not be directing sound adequately. But
@John24ssj could help with comments on that, here's what his speakers look like around his sofa and he's very happy with it:
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