I don’t see why this should be so difficult for the self proclaimed most technically advanced sport in the world. Raising the minimum weight of the car in general isn’t very useful as a 10kg lighter driver will still have 10kg more weight to shuffle around inside the car.
What I think is more sensible: Driver plus seat must weight no less than 85kg (or whatever people think is best). The weight of the seat must be evenly distributed, same density to the material across its entirety (or if there are concerns thats to heavy for safely extracting the driver in case of an accident, make the seat two layered with the inner layer as it is and the outer “weight-layer” removable).
It is true, in most sports it is important to physically be a good fit to the requirements of the sport, but for example in football there are many different kinds of players that can be world class (sometimes because they are different), in ski jumping rules have been amended to better reflect the diversity of the field and in F1 it could be rather simple to make it a bit more fair, then why not?