Going from a mid motor to a hub because you don't have enough torque for hills is going to give you a worse-performer, regardless of how the advertised torque numbers pan out. A hub motor powers thru the axle, so it is effectively a single-speed motor. A fixie, if you will. Think of how your life would suck if you have no gearing and had to live with riding up a hill in a relatively small cog. Thats how the hub motor has to live life. The biggest selling point of a mid drive is the ability to use the gears. What you need to do absent other options is to re-gear it, which you are already in the process of doing.

Doubling up on the power on a single wheel is not likely to be a survivable option long term. Hub motor cassette bodies are bolted on via 5 bolts onto a metal cover plate on the hub motor. That plate can bend easier than a cassette body attached to a hub. Likewise if the hub motor has a freewheel, those threads are not meant to be pulled on with 80-90 Nm of force. I have seen a couple of instances where threads gave way. Ordinarily a hub motor is built with the idea that the drivetrain gets light duty from the rider, so they are not very beefy in this regard.

Plus, for 135mm dropouts I think you are looking at lower powered hubs. Something like a geared Bafang G020 which is only 45Nm. To get to the 85Nm of the big boss G062 you need 190mm dropouts. You could maybe find something from Grin Tech and stick a Phaserunner controller on it for big amps, but thats going to be really expensive.

I would do what it looks like you are already doing: a smaller ring on the front, and if that doesn't do it, convert from a belt+igh to a derailleur and a wide range cluster. The latter would maybe take this project out of the range of cost feasibility since it would require a new wheel build.

Also those dual chainring setups are typically manufactured by a Chinese company under the name Refully. They are fairly new to the market and I would not call them established just yet. I have seen them for sale for a few months, but not too many actual users out in the wild yet.