As polyhive. 1:1 for spring feeding. Think brooding - the bees need water, carbohydrate and protein.
Encouraging them to brood now is a possibility, but remember that a large expanding colony will need to be kept fed right up until they can forage sufficient for themselves. A late spring can mean a lot of feed needed. In the UK there is usually plenty of pollen for 'naturally aspirated' colonies, but 'turbocharged' with extra water will soon use stores unless extra is supplied. I used to supply an in-hive water supply, rather than sugar solution. I reckon it worked just as well, 'cos they had plenty of stores to go at (14 x 12 frames).
A late cold snap, where the bees cannot fly for water for several days can severely stunt the brooding - larvae have a high water content and double in size very quickly.
I've not encouraged early brooding for years. One needs three week old bees for foragers. When OSR blooming was more predictable, it was easier to guess/calculate when spring build up was needed to get underway.
But back to the OP - 1:1, neither fondant nor 2:1 in spring for early build up, so both are wrong. Fondant is only required as an emergency feed, when short of stores. For brooding, the bees would need to forage for even more water! If fondant is fed the bees will take it in preference to honey (higher calorific content) for winter survival - likely leaving insufficient space for early brooding. Need to be starting build up now for early OSR, I would say. Only strong winter colonies will build up quickly of course.
My preferred route was that of swapping in brood (and sometimes bees) to the production stocks, to
boost them (the hives that were to be moved to the OSR). Stopped early swarming of the other colonies, too. Anyone who wants to accelerate spring build up needs to be a thinking beekeeper; non-thinkers are best leaving their bees to build up later. Less potential crop, but safer.
