With no enclosure I would be pretty confident warping due to low ambient temp is what’s causing your print failures. With the top popped my chamber is around 40 °C / 104 °F and I haven’t experienced any PETG warping. My printer lives in my basement, which occupies a similar temperature range as your garage. On my old unenclosed i3 clone I would run into warping on bigger PETG prints.

If you’re feeling ambitious you could try throwing a (big) cardboard box over the whole thing and see if that helps. A few bed fans for good measure could be enough. I wouldn’t treat this as a permanent solution, but it could help you troubleshoot.