Would help to know your dimensions and the materials you’re using in the rest of the kitchen build. However the easy answer is concrete ideally, preferably with rebar. However most oven bases you see are very over-engineered and I’d be very surprised if you had any issues so long as the slabs are laid on a decent solid base with absolutely no voids. The sleepers will do a good job of spreading the weight. There are a number of companies selling sleeper bases to do exactly what you want.
That said you only get to do this once and if you get it wrong then it’s a lot of work and expense to knock it down and build it again. Concrete base would be my advice.