I've tried to model some OWA Yagis using MiniNEC or NEC-2 interfaces, like
Eznec and 4NEC2 (a friend used also AO and YO), but the results are
disappointing, especially regarding the impedance, which is one of the most
important result for OWA.

Really? When I built my OWA's (6 el 28 Mhz, also a 6 el 21 MHz) I used
EZNEC (NEC2). The SWR curve of the final antenna matched pretty well with
the modeling and I didn't need to adjust anything on the final antenna.
I used DX Engineering element brackets which insulate the element from
the boom.

Tor
N4OGW