I have on numerous occasions used a breaker bar or just the proper wrench and placed them in a position pointing down as much as possible...Lay down and get your foot on the wrench while holding on to some part under the car...Some would be amazed at how much more power one has in his legs compared to his arms...I have broken loose many caliper bolts in this fashion...
It’s not your poor maintenance but that of the previous owner(s). Time to replace the Calipers - don’t waste any more time. Do the job right. Good luck FR friend.