1X fussy, 1X greedy advised they need high fibre?


Sorry if this has already been asked but here is my dilemma and I’m not sure what is best to do.

I have an 18 month old cocker spaniel who has always been incredibly fussy, will turn his nose up at food one day, gobble it the next but sometimes will go nearly 2 weeks without eating, turning his nose up at his dry food. He is fed Millies Wolfheart, usually around the 60-70% ranges, changing the flavour to keep him as interested as I can.
The vet has told me he is underweight, but there isn’t much I can do as he isn’t a great eater.

I also have a 9 month cocker spaniel who is incredibly greedy, would eat dust if it was available, and I have been advised she is slightly overweight / at risk of becoming overweight. I feed 20g less than the bottom of the feeding guide for her age/weight and she has very few treats so I’m wondering if Milie’s isn’t right for her.

They have both had anal gland issues and have had to have them emptied frequently which again is a worry at this age.
I have added profibre to their meals, the 18 month old seems to benefit from this, but it makes no difference for my 9 month girl, (also tried weetabix which does nothing for her too). The vet has advised me to put her on Royal Canin fibre (I think its called) but it is very expensive and my limit really is Millie’s with cost.

I’m wondering if anyone has had similar experiences and any suggestions for a good quality food similar price to Millies that could work for both of them, or if I will need to have different brands for the 2 dogs? I know the 9 month old needs high in fibre, but wondering if my 18 month will also benefit from this as he currently has profibre added.
I have tried basically all dog foods from pets at home for the fussy eater and he only ate (occasionally) wellness core and step up to naturals, all other foods he just would not touch (and I persevered as written in the tips here)