I have taken out countless slabs. They are NOT forever, as it would seem. Go ahead and pour a slab - you CAN remove it later if you need.
I have taken them out to pour larger, thicker ones, or to move them a foot, or to move the spa to the other side of the yard, or just taken them out after removing a spa permanently. I had a customer who was doing a large remodel, and had to set the spa aside. They did NOT want to give up using the tub, so we poured a 3" slab in the back corner of the yard, ran power, and they were able to keep using it. A year later, the house was fully done and they were ready to move the spa back. Three guys with a (rented) jackhammer and the slab was history in a half day.
I know it seems like a lot of work, but installing and even taking OUT a slab is - in my opinion - easier than digging and compacting for a sand/gravel/paving tile base, and it is much better as far as settling, shifting etc.
Food for thought