> Do I understand correctly that an Uber driver with a 30k capital investment in a car is part of the petite bourgeoise
In the simple, no debt financing case, that's the most obvious categorization, though much the same concerns that lead to questions about whether those drivers are rightly categorized as contractors might be raised as to whether they are genuinely self-employed independent business owners or rented labor being applied to Uber capital with a weird gatekeeping mechanism.
> and a surgeon working for a hospital who has put 400k toward education is part of the proletariat?
A lot to high income workers (surgeons often very much among them) are joint capital/labor earners of a kind that diverges from the independent small business owner model, because they gain substantial income, often deferred through reinvestment and often mainly in what are held largely as retirement funds, on capital to which their own labor is not applied, as well as selling labor to capital; for that reason they’d generally be seen as petit bourgeoisie, thought somewhat different from the main textbook example.
> Or do you count education as capital?
Traditionally, if it's not been made property that can be itself sold freely in capitalist economy, it's not capital in the analysis of economic class in a capitalist economy. (Looked at from within the system rather than from critics of capitalism, ask yourself: are the returns of education taxed like labor or like capital—that tells you how the capitalist system itself sees them.)