Hey Justin. Thanks for reaching out and welcome to the community!
Great questions, too.
From what I have seen, Teradek Ace or Bolt are top end options for the transmitter/ receiver. Do you agree?
Absolutely. Teradek is basically the only brand I recommend for wireless camera systems under $5,000 in a live production environment, especially if IMAG is being done. In my opinion, you really have no other option under $5,000 other than Teradek if you want reliability and lowest latency.
Also--can I get away with using one of the JVCs as the roaming camera.
Sure, you can use one of the JVCs for a roaming camera. It will be the easiest way to match your other cameras and keep everything consistent.
Any stabilization suggests for those? I've seen some places use that giant harness that has an arm that comes off it but have never messed with those before.
The harnesses you're referring to are usually referred to as stabilizer vest systems or "easyrigs" (which is technically the brand name for one of the most popular options for those systems). I don't recommend easyrigs at all for a vast majority of church environments. They can be more of a distraction and frustration than help in most cases. The only reason I would recommend an easyrig in your case is if your camera system gets over 20 lbs or the operator has to use the camera for a really long time, like 4+ hour stretches without a break. If you're just running a JVC HM620, a battery, and a wireless system then it should stay well under 10 lbs and be very easy to run in a regular handheld or shoulder mount configuration.
I would also recommend staying away from things like gimbal stabilizers because those tend to limit the types of shots your operator will get with that camera. Being fully handheld or shoulder mount will give you the most versatility for shot options and will give your operator the most mobility with that camera.
So I would just recommend looking into a simple shoulder mount kit with a base plate, shoulder pad, rails, and handle.
Hope this helps!