Lots to think about...instead of actually, you know, sleeping.
Bob; my T&G set is 1/2" shank and i still haven't built a RT. I do have that small RT from LV, but it's for my 1/4" trim router. Too small a table in any case. And to be honest, the TS is faster, especially if i do the spline thing. single pass on both edges plus ripping a bunch of 8' splines (160 lengths x 1"...including the kerf= 3 1/2 sheets of plywood= waaaay too much expense).
I would rethink the spline material, Dan. Use any scrap wood you have laying around,including the cedar. The spline doesn't have to be perpendicular grain in this case, a tongue is not. You are going to be nailing every 2', it is not for strength,it is for alignment in this case. This would be the most expedient method, everything on the TS.
Do you have a slotting bit for your router to hand slot the ends? if you do that will save a lot of material too by end joining between soffit joist.
Do you need air circulation in soffit ?
Remember to give the squirrel a 30 day notice.