Friend, I know the lingering doubt you speak of all too well. When my Dante was diagnosed with an enlarged heart and a partially collapsed trachea, my vet said going to a specialist for an MRI would be the best way to tell if there was any sign of damage, despite not seeing anything on the x ray he did. It would have been about the same cost as what you mentioned.
I was very lucky during this time that an old close friend of mine was working as a veterinary technician, although further upstate. I shared his labs, x-rays, findings with her and she reassured me that at his age, an MRI would only be telling me what I already know: that I should start spending more quality time with him. He died a week and a half after his diagnosis of heart failure and sudden fluid build-up from which we could not bring him back in the animal ER. Could I have gone to the specialist and gotten the MRI? Yes. Could I have found out how little time I had left? Yes. Could I have put him on expensive heart medication? Yes. Would that have guaranteed he would live much longer than he eventually did? No.
Unfortunately there will be many "what if"s for you during this time, but they do nothing but force you to relive the worst of it without the ability to do anything to change anything. It is a brutal, senseless, painful cycle. You did what you could, you did not put your love under more tests and medications that would ultimately have done nothing but prolonged the inevitable. Your Batista got to spend his remaining days under the love and care of his beloved, and was escorted into the great beyond with love, in peace. Please take some solace in that, and remind yourself of everything you did, all the time and energy you spent, on being such a good pet parent. I promise you Batista is grateful for that time and for the love, and for the dignity you gave him in his last moments.
Please take care of yourself.