Well, Brassavola, that explains a lot, and likely that should have been the first question asked. Makes a big difference. Ugh on the clay soil. When I first moved here I had an FHA loan. There were no gutters and the ground next to the foundation had eroded. The lender insisted the seller put on guttering AND backfill more dirt around the house. Of course the seller used the crappiest cheap soil they could find, and it was almost solid clay. Took at least a decade to grow things well on any side of the house.
My opinion, and I know it's after the fact... I would have gone with #3 gravel instead of #4. It's washed, and allows for more drainage. I figured your supplies were already ordered and no going back since it was being delivered, so work with what you have. Or get a second load of #3 and some chat, then use the #3 elsewhere, and use the sand around outside perimeter of greenhouse to un-clay the soil somewhat. And that may not be economically feasible.
Leaving the pavers above ground level will help. Also leave them a bit higher than the brick perimeter you're putting on the inside. Maybe a half inch max, depending on the pavers you're using. Put a layer of permeable geotextile at bottom of the base once you've dug. Then the gravel, and tamp it well. Were it me, I would lightly wet it, then tamp again. Then do another permeable layer of geotextile over that, then your sand base and lay the pavers. Don't fill in any gap in pavers. Use the mallet to tamp down each paver to level.
I'm not sure why you're putting a row of bricks around outside perimeter of the greenhouse foundation. Rationale being?? You could always save those bricks and see how it drains, then do a french drain on outside perimeter later if needed. i.e., it doesn't drain to your satisfaction.
Definitely I'll be here cheering you on! It's a lot of hard work, and you don't want to do it twice. Have you figured out screeding yet? Feel free to give a shout out if not. I've never done a greenhouse floor, but have laid more than my fair share of patios, sidewalks and pathways. It was part of the biz.
Here's your first cheerleader squad...
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