Gorgeous guinea pig @unlikelychump and glad things are looking up.
I am a long-term guinea owner and can highly recommend website The Guinea Pig Forum for lots of brilliant advice on all things guinea pig https://www.theguineapigforum.co.uk/
Key things to bear in mind would be:
Diet should be 80% hay, 15% veggies (lettuce, cucumber, pepper etc - be careful about giving too much broccoli, kale and so on as can cause bloat), 5% pellets. They don't need vitamin C tablets in water, they get all the vitamin C they need from their food.
Aim to weigh weekly and do a general check over for each guinea pig to make sure all is well. Pigs should maintain their weight to within +/- 50g. Any loss of 50g + is concerning and should be investigated.
Get some critical care (essentially dried hay to which you add water to create a kind of puree) to keep in the house and give this if you have major concerns and can't get to a vet quickly. It's essential that guinea pigs don't just stop eating.
Most vets specialise in cats/dogs - very few are actual exotics specialists (guinea pigs, like reptiles/birds etc, come under the 'exotics' heading). However, many cat/dog vets do have a decent amount of experience in treating small furries and prompt treatment can make a huge difference. Over the years I have taken various guineas to our (excellent) general vet for all sorts of things - tooth problems, eye problems, urine infections, respiratory infections, etc etc. Yes sadly there are times when nothing can be done but I have known many other times when a simple course of antibiotics has sorted an issue and the guinea has bounced back to health.
Hopefully it will be onwards and upwards for your piggie now but for anyone else reading this, please always take a small furry to the vet if you suspect illness, and do it sooner rather than later - it can make all the difference.