I'm not a beardie vet or a vet at all- but im a registered nurse with a little experience in heart problems for people. Management of heart failure in people typically is ongoing diuretics and fluid restriction, it's a life long balancing act with ups and downs
I'm not sure if its the same in dragons but if it works like it does in people then i can offer maybe a bit of conservative advice: it's not going to necessarily be cured, but it can be managed long term without having to constantly aspirated fluid; it does seem like a possibility that he may have a poor heart due to genetics, possibly a "lazy heart" maybe ?
I dont think you will have to euthanize him if he's thriving otherwise, just try to keep careful limit on fluids and diet. A hopeful outcome can be that he can just be on oral diuretics and stricter diet long term. I would also weigh him a couple times week to keep track of the fluctuations as this could indicate increase of the fluid.
Try to monitor the behaviour, appetite, and weight week to week and see if you can get a good balance ongoing. Keep a diary and perhaps discuss with your vet ongoing over the next few months to see if you can sustain a fair baseline for him just by these patterns and adjustments in his diet/intake
if he has healthy liver and kidneys then he shouldn't have issues with ongoing diuretics for a little while but perhaps with enough time and tweaking he can wean off those too