which sub to get

You may only need one sub but it depends. It is a practical compromise though. Two are really needed for optimal results in all regards from a sweet spot. That's why any full range stereo speaker has woofers. 4 even maybe if you want the right bass throughout the room, not just the sweet spot.

I have those same Pioneers matched with my Cain & Cain Abbys. My
room is 17x35 with cathedral ceilings. Not huge bass, but fast and musical.

A properly set up T5 will work. I suggest that you use the speaker outputs to the REL rather than going through any other crossovers. Proper set up ( location, phase, volume, crossover setting) ) is crucial and differs from other subwoofers. I would suggest you set the crossover point at 55
Herts to start with. Don't be afraid to place the sub in a corner, facing across the room. My son uses a T5 with very good results

I agree with Mapman and Bob Reynolds. And also remember when I was thinking of adding a first sub to my stereo and had no measuring capabilities - or understanding, for that matter. So can relate to the OP maybe getting overwhelmed.

I started adding a REL Storm III and loved it. Paired with B&W 804S. The speakers were used full-range, just like REL recommends, and the sub augmented the speakers low frequency output.

Then I got into acoustics, got myself stuff for measuring, experimented, etc. Eventually I bought two Rythmik 12" kits and built very heavy sealed subwoofers. Rythmiks are outstanding subwoofers, and much cheaper than REL.

Initially I set up the Rythmiks to augment the low end and measured and tweaked a lot. My amp is tubed, so I then experimented running from the preamp (also tubed) to each SW crossover, and feeding my main amp everything above 80Hz. To my surprise and despite the lower quality of the parts inside the included crossover, I liked the sound better. I think both the tube amp and the 6" woofers are happy not attemting to reproduce below 80Hz.

I definitely recommend two smaller subwoofers over a larger single one. And by adding even more subwoofers you can make the low frequencies response smoother across the room. See Earl Geddes papers on this.