With the old shape Nissan Leaf, we can squeeze a slender adult between a modern booster seat and rear-facing baby basket, both with seat belts only not isofix which gives a crucial few cm. We've done the same with the Tesla M3, and while on paper this should be easier it doesn't seem it. The shape of the cushions must play a role here? Neither is something you'd want to endure regularly or for an extended journey.
It would be a safety trade off, but we may get an older model booster seat without any side protection to make this more feasible.