(Just finished S&D, starting Halflings.)
Personally, I tend to rely on event buildings for mana. And that’s what I’d recommend to anyone else too. Several Frost Carved Trees from Winter event got me through Woodelves and most of the S&D. I did build some Willows and Libraries respectively, but no huge numbers.
From the Carnival event I got the set buildings and those boosted my mana production further (Revelers Fountain gives a lot when linked).
Another thing that helped me was a strategy Soggy shared: when player deletes Woodelves settlement, the space can be used for temporary Willows (which are then deleted as player builds the next settlement). It builds up significant mana buffer that comes very useful during the S&D. The same is repeated after S&D, just with Libraries instead of Willows.
Anyway, at the moment you probably don’t have much choice. So I’d say “upgrade” Willows to Libraries and add some Libraries. If you implement my suggestion above, you’ll be able to significantly reduce your Libraries after next big event - assuming there’ll be some good mana building(s).