As William said – Talk to your Doctor.
Your diet as a person with CF has been designed to cope with your gut not absorbing food well. With starting TK that changed though some damage may persist – take time to heal or even be permanent. Fibre is important in the functioning of a normal gut but how normal is yours at this time. Other drugs can also cause constipation. Dapagliflozin is a diabetic drug of the SGLT2 inhibitor group. It prevents the reabsorption of glucose in the distal kidney tubules causing increased urine production and loss of water. This group of drugs can cause severe dehydration as the gut absorbs water to replace that lost in the urine. Increased fibre in the diet does retain water and modulate the softness of faeces – makes hard faeces softer and fluid faeces thicker. Metamucil is good – I take Benefibre 1 heaped desert spoonful with my breakfast muesli. Apart from increasing the volume of poo I know of no nasty side effects.
Trikafta by changing the dynamics of Chloride transport may be expected to change the transport of salt and water across the gut wall. Therefore we should expect the consistency of poo to change. How much it will change will depend on the condition of the gut itself – a pwCF who is more than a few years old is likely to have gut damage that may or may not be reversible. Globlet cells may have been lost so reduction in the amount of protective mucous may be present. Scaring from repeated infections too.
We may expect TK and the other modulators to improve gut function but there may be some damage that is permanent. But increased Fibre should help most people.