Hi GrandpaJay, welcome to the EBR Community! I'm trying to understand your needs a bit better, you started off asking for a more powerful ebike with 500 watt motor but also mentioned the Hill Topper kit which only offers 250 watts. Were you planing to use this kit in conjunction with an ebike or add it to a regular bike?
I've tested the Hill Topper and enjoyed its low price point and simplicity of operation. The velcro button you described is easy to mount and use but doesn't offer pedal assist or much variation in terms of speed and power. It's really the opposite end of the spectrum from a 500 watt pedal assist or throttle mode ebike.
I'm glad you found the Volton Alation 500 because it offers an excellent combination of low price, power and weight distribution that's kept low and centered on the frame. Note that this bike does not have the larger radius 700C wheels that your Hill Topper kit may have. Could you please clarify if you already have a Hill Topper or are just considering one because they do offer smaller sized wheels that would work with the Volton Alation.
Honestly, I would go one way or the other. Either use the Hill Topper with a standard bicycle or go for a fully featured purpose built electric bike. By combining two options you would end up with a heavier, under powered, over coplicated machine and your return on investment (money spent) would not be optimized. You would be much better off to get a second battery for an ebike than combine it with an add-on kit. I hope this helps you out and I'd be happy to expand here if you've got more questions!
Considering your weight, the 48 volt option of battery pack on the
Volton Alation 500 would be worth going for. You might also consider the 48 volt
City Commuter by Pedego which does have the larger 700C wheels and 500 watt motor. And another option would be the standard
Pedego Cruiser which only has a throttle (no pedal assist) making it less complicated to operate and more durable. It's also less expensive and comes with a 48 volt 500 watt option. The downside to these latter two bikes will be the rear mounted battery pack vs. integrated downtube.