I'm considering buying a cheaper large frame bike. Probably something generic just to use with an indoor trainer during the cold months while I save up for a decent road bike next spring. I mean how rigorous can riding a trainer be on a low quality bike?

As long as you can get enough seat post extension to get the proper leg extension, and get the bars far enough away and high enough so that you are in a good position - it shouldn't matter too much on the trainer as handling will not be an issue. Look for an XL to XXL (or even an XXXL) "cheaper large frame".

Think of it like a car. Even if you bang your head on the low ceiling of the car, smack your knees on the steering wheel, can't get the seat back far enough for your legs to be in a comfortable position, feel a bit cramped in the driver's seat, have difficulty setting the mirrors so you can see behind you, feel like you are driving one of those tiny clown cars in the circus - you can still drive the car around even if it isn't designed for your height. Bikes are a lot like that for tall guys. You can still ride around and make the bike work for you even if the fit is full of compromises for optimal handling, power production, and efficiency. People have been doing that for 100 years+. The body adapts and you are still getting exercise. That's the important thing for you right now.

Having the luxury to get a bike that fits you like a glove, and is the optimal fit/comfort sometimes has to wait.