SLH-DSA signatures under the SLH-DSA-{SHA2|SHAKE}-128s parameter sets are 7,856 bytes (not 7,872).
Fixed, thanks!
Is there any reason you don’t use the optimizations (e.g. WOTS+C, PORS+FP) in the fallback SPHINCS+ instance but you do use it in the compact path?
No particular reason. There’s quite a range of parameters that you could use for the fallback. If q_s = 2^{30} is fine for your application, you can get 3440 byte signatures (WOTS+C, PORS+FP).