I’d be happy to try.
Let’s start with the speed sensor issue, since that would be the cheapest to resolve. As I mentioned, I use the LeMond sensor, which is a proprietary ANT+ sensor that somehow paired with 2 Edge 500s for me, but won’t be recognized by a 3rd or an Edge 800. I went at it from several directions, but eventually just dedicated a specific Edge 500 to trainer use. For that reason, I wouldn’t suggest it.
I was likely one of the first ones to use a 3rd party (Bontrager) speed sensor with the Revolution to pick up the flywheel magnet. This was in the very early days of the Revolution. It was an inelegant solution (as I had it installed) which required extra stuff to be mounted to the bike that weren’t used during non-trainer riding. Once I got the LeMond sensor somewhat working, I abandoned the Bontrager sensor. Now that I think about it, I could have done it better. I think if you zip-tied the speed sensor to a piece of flat, 1/8"-1/4" plastic that bolted to the WattBox mounting tabs on the Revolution, you wouldn’t have useless stuff attached to the bike. Might take some work to get the alignment right, but the plastic would be easy to work with and cheap. The sensor would also be protected from the elements, which might extend its life. The concept is still rolling around in my brain, but it might be the direction I go if the LeMond sensor stops working.
No matter how it’s mounted, the sensor needs to be properly aligned with that magnet on the flywheel. The battery needs to be good. Sounds obvious, but that’s our starting point.
On your Garmin GSC10 speed/cadence sensor, remove the battery. This will prevent it from pairing with the Garmin head unit. This is temporary until the other speed sensor is set up.
On the Garmin head unit, hopefully you have the ability to create multiple bike profiles. If so, I’d create one named “trainer”. Then I would select to pair up a “speed-only” sensor. If everything is aligned, it should pair. As a good practice, I would write down the ANT+ ID of the speed sensor for future troubleshooting.
If the 3rd party speed sensor is working, you can install the battery back in the GSC10 and it shouldn’t interfere with your speed readings while you’re under the “trainer” profile.
I use my power meters for cadence, which is another feature they provide. I imagine you’d want to use the GSC10 if possible. To do this, I would see if it would pair as a “cadence only” sensor. It may mess up everything you just did, requiring you to repeat the process, but it’s worth a shot.
I wouldn’t completely disregard the crank or pedal-based power meter option just because you have a Power Tap. DC Rainmaker has been known to run 3 or 4 (crank, crank arm, pedal, and wheel) simultaneously for comparison purposes. What crankset do you have? Slightly used ones pop up all of the time on eBay and other outlets for relatively cheap, which should fall within your budget. Keep your eyes open and you might catch a deal.
That said, zPower will likely be closer to reality for a direct-drive trainer like the Revolution than for a roller-based trainer. Mainly this is because a whole lot of variables in setup are removed. In addition, the trainer’s resistance isn’t based on a fluid or magnetic unit, which can fluctuate significantly over time. The Revolution can be affected by temperature, air pressure, and obstructions to the fan, but not as significantly. Overall, I’ve found the power profile to be pretty consistent over time based on multiple LeMonds, their indicated speeds, and the powermeter-measured wattage.it took to maintain those speeds.
Hope this helps, and let me know how it works out.