Hi NoClue
Firstly welcome to the forum Rhys
The Welsh contingent on the forum will be better suited to answer parts of your question......with local knowledge.
I would suggest that if the river is producing SeaTrout .....then they are but a Brown Trout that has been on its holidays to the seaside

.......so Brownies must be in the system.
There's also a possibility of Grayling too dependant on which river......and these guys will keep you occupied through the winter months.
If at some stage you find the 'want' to try for a 'silver tourist' and odd time or two .....simply buy a one day or one week licence.....and it still wont total the cost of the 'Migratory' permit. Then maybe next year you will have it a little clearer in your mind the direction you want to go.