Oh and also maybe worth asking for (if appropriate and doctor agrees any medical need) a h-pylori stool test. This can also cause gastro issues and is easily solved with antibiotics.
I sense an anxiety and maybe a bit of disbelief that your scary feeling symptoms can be something ‘trivial’ like GERD. I absolutely scoffed when my doctor told me my constant regurgitating mucous and breathing problems and pains were GERD and post nasal drip. I felt 90. I wasn’t sleeping, socialising or exercising and felt sure it must be something terrible.
i’d had long covid and was convinced it was my lungs.
As it happens, as I dealt with the reflux and low B vitamins I felt so well that I never even pushed for a lung scan. My breathing and cardio vascular fitness, now, tells me I don’t need one. I fixed the issues and the anxiety naturally subsided.
Honestly, the likelihood (particularly as you state you’ve been extensively tested) is it’s something fixable.
Think of yourself like a car. When they stop, it’s usually a lack of petrol or a worn down component and not a full engine failure. Sometimes it’s the latter - but you’d check the most likely things first.
You should of course note all changing symptoms and continue to seek medical advice about these, but there are plenty of ‘easy fixes’ which can cause such symptoms - which you can try, mentioned on this thread.
i hope you get to the bottom of it and feel better soon .