I have read people having mixed results with biochar so I generally stay away. If you already have healthy soil it seems to be one of those amendments that does very little.

I like to prepare beds in the summer or fall ...to be ready for spring planting early. I would go ahead and add the biochar with the enrichments now and get things 'charged up'.

I think the sooner the better. It gives it the most amount of time to be integrated into the soil's structure and biology so it can 'do its thing' whatever that is.

The only thing that doesn't fit this approach is very leachable nutrients like nitrogen fertilizer which will mostly leach away before spring.

The biochar prevents leaching. 1 gram has the surface area of 2 tennis courts or more. This is the charging issue. Soaking it in water and compost for a few days will charge it, where dry uncharged char will have huge areas of surface to soak up the nutrients and biolife currently in the soil. Its takes some awareness of its porosity for success. Of course mother nature takes over in a few years if you make a mistake. If its high quality (high temp pure carbon) the cation exchange number can easily be 200+. I think mine from my yard test was like 8 or something last I tested it.