I would wait another week before deciding whether or not to re-treat with the Waterlife Myxazin. During this time, I would feed goldfish pellets (easier to digest than the dried bloodworm / tubifex and possibly even the pre-soaked flakes as less likely to expand in the gut) and peas, perhaps ~66% of meals being goldfish pellets and ~33% of meals being pea. (Cucumber is ok but has little nutritional value.)
For the pellets, some options might be Dennerle Goldy Booster, Vitalis goldfish pellets, Fish Science fancy goldfish sinking pellets, or Hikari goldfish staple or Hikari fancy goldfish balance. Although the latter 3 don't necessarily market themselves for adult single/comet-tailed goldfish, they ought to be fine. If any of the pellets are not sinking ones, then best to pre-soak them for an hour or so before feeding them. [Most medicated food is only available in USA.]
For the pea(s), blanch to remove the shell easily, then chop each half of the pea into small morsels and feed this.
This combination of food ought to be about the right balance to ensure that food passes through without blockages but without erring into 'diarrhoea' (sometimes a fine line with constipation / 'diarrhoea' and thus empty casing being passed in fish).
Do ensure that water quality remains optimum (0 ammonia, 0 nitrite, 20 max nitrates or no more than 20 above tap water levels) throughout this time, and perhaps do small (10%) daily water changes and run carbon through the filter to ensure the previous course of Myxazin has gone. If, after another week, the situation still hasn't changed, then it would be worth considering medication again.
Thank you so much for taking the time to reply! I just fed them a few peas and will do another water change tonight, will keep an eye on my poorly fish and the water parameters. I noticed another thing this morning when I put the tank lights on. I can see tiny white worm-like creatures floating/swimming around, they are thinner than hair and they are 1-3mm long, super tiny. Maybe they have always been there just never noticed them before as they are really hard to see. The larger ones are curved like u or c shape and I can see that they are alive as they are wriggling a bit (I have really good eyes :) but they never "stretch out", they are in "u" shape all the time. As they are curved they look like tiny dots but if I look closely I can see they are curved worm-like creatures. Also, the sick fish has one white spot on its tail again, looks like an ich spot. I know that I shouldn't be able to see ich or most of other parasites swimming in the water but what can that be? I have googled about white tiny worms but I couldn't find anything about u-shaped worms or parasites. My fish behaves normal, not itching etc..If it is ich then I really don't understand how did they survive 3 ich treatments..Can it be skin flukes instead and that's why the ich treatment didn't work? Can it be nematodes? But I think nematodes look straighter, also I can't see anything crawling on the glass. Seems like my struggle never ends :( I guess I need to wait and see if my fish will get more spots.. and then treat accordingly. But the little white creatures worry me..what can they be? Thanks again!
I'm assuming, from what you're telling me, that the worms are not actually attached to the fish? Is there anything attached to the fish that you can see? (This helps figure out if a parasite is likely.)
At the moment, I don't think the one spot is indicative of whitespot/ich but, if more develop, then it likely is. There would be no harm in having a whitespot treatment at the ready, in the event this (ever) develops and treatment required urgently. This article should be really helpful too: https://www.fishkeeping.co.uk/articles_88/white-spot.htm