Not been to Skolholm myself (stayed on Skomer several times). When I looked into it I gather with Skokholm there’s more chance to be storm bound so you have to plan for that issue both for arrival and departure.
One thing I would say is I saw a tweet by Welsh Wildlife Trust saying they were looking into how they will open up accommodation this year. There is a chance accommodation may not reopen fully this year due to COVID restrictions (multiple households sharing facilities) so 2020’s bookings may roll over into 2022 instead of this summer.