Hi Thomas,
You can wait until your first layer is fully cured before pouring the next one, but you will see a line between layers. If you want to minimize the line between layers, you will want to pour the next layer when the previous layer is in the gel phase. This article explains more: https://resinobsession.com/resin-frequently-asked-questions/what-is-the-gel-time-of-resin/
As for the resin, I have two suggestions. Both can be poured 1/2 inch thick at once.
For pours of 3 ounces or less, the Resin Obsession super clear resin is what you want: https://shop.resinobsession.com/collections/resin/resin-obsession-super-clear-resin
For pours of 3 ounces or more, the Resin obsession deep pour casting resin will work well: https://shop.resinobsession.com/collections/resin/resin-obsession-deep-pour-resin
Both cure very hard and clear.