At this point (finished slab) you need to investigate conduit size.

It is pretty rare to 'future proof' conduit though (by running larger than required) without a requirement to do so by whomever is paying the bills.

It is possible to groove out slabs to add more conduit, or even tunnel under them.

You also might want to look at the current/voltage requirements for the units you are looking to install. Tankless units use a lot of amps and would require a large wire size.

Yeah I get the impression it isn't going to happen. The conduit is minimally sized - looks like the two wires going through them barely fit themselves. Why do they cut this corner? It seems pretty ridiculous.

" Why do they cut this corner?"

Using the minimum code required conduit side is not cutting corners.

Very few electrical installations have any provisions for 'future proofing.'

Larger conduits gets significantly harder to bend.
You cannot use a lever and a hickey but must switch to hydraulic benders, and then to factor made bends.

Hydraulic benders are pretty common in larger commercial work, but they are painfully expensive and require different dies for every size and type of conduit they are capable of bending (and on larger sizes still have a decent number of collapsed bends, AKA 'kinked').