I agree with Merlinextraligh on rank ordering choice. We too had TRP Spyres on our Co-Mo and while I loved them over caliper brakes, I found that the return spring on the rear caliper was not strong enough to overcome normal cable slack that you get over the long cable which required me to add a boost return spring which resolved that issue, however, I also found that I had to constantly futz with adjusting the pad spacing. I imagine that the HYRD version would mitigate the latter. Last year I upgraded to full hydraulic and I'll never go back. Braking control and feel is like butter and I haven't had any adjustment futzing to contend with.
That said, I get it, outfitting hydraulic can be a challenge to find the right shifter/brifter's and dealing with fluid and bleeding issues poses some overhead and may limit transporting options..
Not being a braking expert I'll offer the following as nothing more than a point of view. I am sure others on the forum know better. Braking power and stopping power is stronger in the front. On half bikes, we tend to rely more on the rear brake so we don't endo over the bars. There is more cable loss in the longer cable run to rear so we tend to want to beef up the rear brakes. On a tandem, the chance of an endo is almost non-existent and you can (and most do) rely more on your front brake. You mention using a larger disc in the rear. What is behind this choice? Is it to beef up the rear braking force or to dissipate more heat? Either way, given what I am saying I would contend that the front would benefit more from a larger disc. We are a 400lb team and have 203mm discs front and back and have plenty of stopping power, we live and ride in a very hilly area but the longest desents are only a couple of miles so we don't have to worry about sustained braking issues.
In the end I am sure you will be happy with the Spyres and you will get perfectly acceptable braking performance with your current thinking. Going with a larger disc may limit sourcing it, but not insurmountable.