If you want to genuinely do right by the fish you need a much bigger tank. Goldfish should be nowhere near a 10g tank, 3 goldfish certainly shouldnt be. They should be in 50g minimum, or rehomed if you cant accommodate a suitable home.
Your efforts would be better aimed a cycling a more suitable home they can live out their lives in.
There are many types of micro-orgsnism that consume ammonia ultimately leading to nitrate. Some operate at lower pH, but i tend to agree these may take longer to develop.
Your issue may be more related to low carbonate hardness (
KH) than pH.
KH is needed for the nitrogen cycle to function. If you have low to zero
KH your tank simply wont cycle. Do you know your
KH? If you dont have a test for
KH your water company should be able to provide the information, often online.
KH can be added into an aquarium through crushed coral in the filter or a piece of cuttlefish bone in the aquascape. Be aware though that raising
KH will also raise your pH. And as you have mentioned higher pH will make the ammonia more toxic. Your ammonia levels are quite high and 3 waste heavy goldfish in such a small tank will make the water difficult to keep safe. In addition to being a more suitable long term home, a bigger tank, more water, will dilute the waste and make cycling a much safer prospect.