Honestly, spending over $200 on a "legal version of the suicide cord" is the silliest thing I have ever heard. I could see using that if you were stuck in a rental unit or something.
The even worse part is the high probability of misunderstanding something about this complex system: your load's requirement, what the donor circuits are able to provide, or what other loads are also on the donor circuits and cause breaker trips. I would expect this to fail because of being blindsided by technicalities like this.
For the kind of shekels they're asking for that "legal suicide cord", you could pay an electrician to run the circuit your load requires, and then you'll have exactly the right thing.
Or if you are competent to DIY, you could do it yourself for far less. About $60 in parts, the duplex QO being the oddball.
Even for less than the $200, you could install a subpanel in the garage, and take some of the pressure off that overstuffed (but excellent QO) main panel. I gather the 40A is your range. Do any of those 30A circuits already go to the garage? Dryer, water heater, that kind of thing?