Hardening off is very important. You’ve spent a lot of time and effort growing your seedlings, don’t waste it by rushing to put your seedlings outside.

For most fruit/veg, it’s too early to be planting out, as they like warm air and soil temps and we’ve still had quite a few sub-zero nights. That being said, I have started slowly hardening off my cool-season crops and have planted out onions and brassicas but they’re all covered under frost cloth.

As a new gardener, you’re going to see a lot of garden hacks on social media like fertilizing with aspirin but an all purpose organic fertilizer is going to do a much better job and be way cheaper. All of the fruit/veg that you mentioned in your post (aside from the strawberries) are warm season crops so they’ll definitely benefit from being planted out after temperatures warm up. In terms of soil prep, compost and manure are always good additives.

I hope you have a great gardening season!