I think I've settled for this option of roadlink + mtb cassette.
On wolftooth website they specify that an 11-40 is ok with a GS derailleur as long as the front teeth gap is less than 10, rather than my current 34/50 compact one, will it work anyway?
What about getting a roadlink + mountain cassette ($)
or
tanpan+mountain RD+ mountain cassette ($$$)