Hi there,
I'm having similar problems and understand how you feel. Luckily, my bunny is insured (I'm in the UK) so we've been able to do various scans, but they haven't shown anything other than fluid on her lungs. This week though she got far worse after we increased her diuretics so I've since stopped them and we haven't had anymore episodes.
My other bunny who has been diagnosed with an enlarged heart, has been on medication for years and we only x-rayed him and my vet and I worked out a plan to trial medication. Which worked wonders for him and unfortunately hasn't for my girl. She has been described as incredibly rare though and has multiple vets stumped
My cardiac bun was actually only one when I noticed his symptoms. They were *very* subtle in the beginning that the vet didn't believe anything was wrong until we x-rayed. He was very lethargic, I could tell it wans't normal because my bun who is 4 years older was far more active. He would sit upright rocking and staring into space. He also would make snoring, raspy noises when he was resting (not asleep) and when he was grooming or eating his cecals. The vet has heard the noise and said it's coming from his larynx, but I think it's all connected to how large his heart is. Also, he needed antibiotics for something else and he suddenly went very thin and my vet could 'hear' his heart was enlarged -whatever that meant.
They can have 'thickened heart muscle' which is normal for their age, but it also could be 'borderline enlarged', which is what my other bunny apparently is - though she's almost 9 and it's considered normal. Snoopy's heart was actually touching both sides of his rib cage
Unfortunately, there is no definitive way of saying whether it's enlarged and what the treatment might be without the specialist scans, but they were out of my reach even here in the UK too. Heart specialists are extremely hard to come by, so we had to basically guess with Snoopy based on the x-ray. Luckily for us, we guessed right. My vet said trialling fortekor was safe and we started on a very low dose - it has a huge dose range so you can increase it later. I had this tablet that I cut into 10 tiny pieces once a day. He is also on pimobendan, half a tablet (for dogs) twice a day. I think that has made the biggest difference for him and dogs are living for 'years' my vet said on this, so we think that's why Snoopy has lasted so long. We weren't sure if he had fluid on the lungs. When it was v hot I would occasionally hear noises. My vet gave us furosemide (a diuretic) for any time he appeared laboured, but he really never did so I stopped using it. If your bunny has laboured breathing then it might be worth trying a diuretic twice a day. Furosemide is supposed to be the best one. I was told fortekor, pimobendan and furosemide were the key 3 drugs for heart failure. We did have no choice with Snoopy but to trial medication, we couldn't get an exact diagnosis.
So even in the UK we are still guessing a lot with bunnies. I will help if I can, I can tell you what helped Snoopy and that he's on the meds above. Fortekor is my vets preference, but we initially started him on pimebendan and furesomide. So I would say any of those would be a good starting point. If you hear any crackly breathing or your bun looks laboured (rapid breathing, holding their nose in the air etc) then I'd definitely try a diuretic too. I feel really frustrated in the UK by the lack of resources for bunnies so I can't imagine how you must feel. It's heartbreaking isn't it? x