Not really sure where to go from here: cat with respiratory distress attacks, ruled out asthma and an infection - need advice on how to manage condition/if further testing is warranted

• Species: Cat
• Sex/Neuter Status: Male, Neutered
• Breed: Manx Mix
• Body Weight: 20lbs, 7.5 Body Scale
• History: Outside of respiratory issues, he has experienced mild constipation (saw vet, was put on laxatives), mild gingivitis (dx by vet), and dandruff (just seen at home)
• Clinical Signs: Respiratory Distress Attacks, Wheezing
• Duration: One Year (Since I have had him)
• Location: New England, USA

My cat was previously owned by my ex who insisted he didn't need medical attention, thus the reason why he was only seen recently once I left and got ownership. For the record, I did not steal Thunder, ex gave him to me willingly (Thunder hated him).

I got a video of Thunder's respiratory attacks and sent it to the vet prior to my visit. Upon arrival, they agreed with my assumption that it was most likely asthma.

Thunder was given an x-ray which revealed "either fluid or fatty tissue in his chest cavity surrounding his lungs and heart", and "mild lung irritation inconsistent with asthma".

Vet then did a blood-based test (unaware of it's name) to rule out heart issues - came back negative.

We were given a loose diagnosis of either respiratory issues caused by his weight, or a respiratory infection, and sent home with antibiotics and a diet plan. Both have been implemented, antibiotics are done, diet is still ongoing.

Vet said she was hesitant to prescribe any steroids due to the possibility of any kind of stress on his heart, though she believed they may improve his condition it was overall not worth the risk.

I was advised if his condition improved no further testing was needed (financial concerns).

Post-antibiotics (full course), his condition improved drastically but was not eradicated. He went from having respiratory attacks once daily, sometimes twice daily, to about once a week.

I'm wondering what the hell is going on - if the antibiotics helped so vastly, why is he still having attacks at all? Is it possible he had a respiratory infection this entire year, which the antibiotics resolved, and his weight/possibly environmental factors are still triggering episodes? Is further testing/another appointment warranted?