If the front and rears are the same set up, have a look at the cable, inner and outer. Thats probably where your issue is. Routing or corrosion.

IMO; Shimano long drop calipers are pretty much as good as their standard ones.

Give the TRPs a go certainly; but I never had any issues with the Shimano calipers.

Your issue could be cable routing; are all the cable outers secure in their mounting points; including at the lever?

Then make sure your pads are hitting the rim squarely. This can take a bit of fiddling to get right. You might need to remove them and sand them flat if they’ve been hitting the rim on the piss.

Other than that; they should work. You won’t be able to lock the wheels initially though, if it’s wet; the pads need to clear the water from the rim first.

I initially used a cheap cable set of Jagwire outers with galvanised inners, I’ve since swapped to Shimno outers and SS inners, the cables are secure and the pads are hitting square on, the pads are Koolstop Salmon and aren’t worn. It has to be said that I’ve been riding in all weathers whereas my summer bike has never been out in the wet. It’s not that I can’t stop, the front brake is adequate, it could be the pads don’t suit the rims, the Shimano pads in my XT 780 discs brakes on the XC bike felt really poor, it was only after swapping to UberBike race pads did I get really good performance.The TRP brakes have cost me less than the R650’s and the black finish will match the bike better, and hopefully a give marginal improvement in braking too.

IMO; Shimano long drop calipers are pretty much as good as their standard one

I concur absolutely. R650s come with the same pads as my Ultegra calipers too. I feel no difference between R650, 6600 and 6700. Mine are pulled by Cane Creek levers rather than STIs, and I can lock the back wheel as needed, even in the wet. Either people with poor braking are a lot heavier (me, bike and pannier are about 80 kg), always using the hoods rather than the drops to push the levers back (which has a lot less mechanical advantage) or pad set up is not optimal.

I suspect it to be the second of these. Try braking from the drops really hard and see if you still have the same problem.