How much work and what tools required for DIY?
Loads of work. A tree trunk is going to be very heavy until you’ve managed to hollow it.
For DIY if you’re not handy with a chainsaw then you’d be looking at you’d be looking at an Adze and a box-set of negro spirituals. Get your wife some Aviator Sunglasses, and rifle and some chewing tobacco so she can watch.
Alternatively an angle grinder and some arbotech disks (basically chainsaw teeth on a metal disk) are good for removing volumes of wood. PPE up though I’ve had disks fracture and the pieces come of off bullet-fast.
Where are you siting it? A chainsaw-ist could more readily cut straight through rather than hollow out a trough – effectively making a raised bed if you’re siting it on soil rather than a patio.
For quick and easy and a rustic look though you could get some ‘Slab’ from a sawmill – effectively the first slice of a log so you’ve got one flat face and one natural bark face – then fix those pieces together like a traditional planter / raised bed.