I take Apple Cider Vinegar tablets every day. They sell them in capsule form, and fast dissolving tablets. I got mine from Costco, and they’re great! But I’m sure you could find them more easily on Amazon (I don’t use this product. It was just the first one I found).
Edit: I realized I never answered your question. Sorry for that. I meant to say that I’ve taken them at every possible time of day, and I haven’t noticed anything at all. I have acid reflux issues as well, and it hasn’t bothered me. However, I only take capsules rather than a shot of the liquid form.
Sensodyne is the name. Novamin is the stuff he's talking about. I have it, you can ship it on Amazon without prescription. I'm sure on some minute level it does help as much as people claim, but personally it didn't really help at all. The reason it isn't available in US is assumed to be because it wasn't approved with the label that it can "repair" teeth, so they just cut it.
https://www.amazon.com/dp/B008VPSTOA/ref=cm_sw_r_cp_apa_i_kFOgEb5MC5R3H