Hello,
I can say something about that because in August I dug a hole for our Berg Inground trampoline.
Basically, it's really easy - how deep and big the hole should be is all in the assembly instructions and on the WWW.
I think the seller told me that a diameter 10 cm less for the pit than stated would be better - you can always trim it later.
I dug the pit by hand - lots of stone, all with a pickaxe.
A mini excavator is clearly an advantage there...
Such an awesome thing, an Inground trampoline - I can recommend it.
I would put the swimming pool directly next to the trampoline and not in the ground - then you can jump straight into the water from the trampoline - what fun!
Also, you can put another trampoline on supports next to the Inground one, then bottom to top - friends of ours have that.