Should work... this is where si.shimano.com is your friend.

2018-2019 SHIMANO Product Information Web

Looking at the compatibility chart for e-Tube it does show that SW-R600 should work as shift buttons. I've also heard of people cabling them directly into the junction box, which suggests the same.

The two thing to keep in mind with these is:
1. The cables are short, they are only designed to be run from the stem to the levers, so you may not have enough cable to get from the brake levers to your junction box. If you're handy with a soldering iron, you could splice a longer cable, or possibly open the shifter and solder directly to the board. I had to splice some e-tube cables to run Di2 in my bike without modifying the frame, it's not that bad, just two wires, similar to CoAx cable, but much smaller.
{edit** I do realized that there are junction tubes to join multiple wires, but didn't think it would be a good solution given that this will likely be under bar tape or just outside of it.

2. They don't have the 3-port hub like the drifters do, so you won't be able to run sprint buttons or anything else off of them. Sprint buttons require that they are plugged into brifters and can't be plugged directly into the junction box, so that's just not going to happen.

All of that being said.... is there a reason you're opposed to Shimano's own hydraulic Di2 levers or do you already have the TRP's in hand?