I'd give the plants several, light side-dressings rather than one, large feeding. Too strong a fertilizer can burn the roots.

If you have compost you can't go wrong with that.

I would rather use 10-10-10, twice the amount of 20-20-20.
You will end up suppling the same amount of fertilizing power but in a less concentrated form, so it will not burn the plants. Better yet, dump a lot of manure/compost combo, sold at HD or similar stores. This will be better, as it provides nutriens in diluted form, it is organic, and at the same time amends the soil too. But then, it might cost a little more.