score:5
The mistake you are doing in your considerations is the segregation of Cuba apartments depending on renting platform into rustic/medium/premium et cetera.
The truth is that you can find a fabulous villa on Miramar (1, 2, 3)
absolutely by any means: from locals, on forums, just on-site, from AirBNB, from renting sites. Just google for casas particulares and you will be flooded with the results.
It is true that Internet in Cuba is (was) uncommon, but the situation is changing rapidly: finding Internet-enabled house in Cuba is not like finding a unicorn anymore. They exists and moving to AirBNB looks very natural for modern hosts.
However, the more hosts are listed in ABNB, the more competition between them and faster they are booked. That's why the most marvelous private hosts, the true diamonds, are not listed on ABNB, but passed from mouth to mouth and they are booked in advance for years. For example, tourists in our community often share kind and lovely Cuban hosts hand-to-hand, along with personal recommendation.
My advice: do not stick to single way, try different means and approaches. The diamond is out there...
P.S. Pay special attention to reservation commitments, as Cuban hosts (of lower profile, dunno about premium ones) are widely known for their booking unreliability. I was reported oftentimes, how travelers arrived to their booked apartments, that were already re-booked by other travelers, who offered more. So up-front payment is highly appreciated, as it can be a steady assurance of your booking.