*ad bankruptcy: yes, please. But with the chance of the teams cancelling the sponsor immediately.
I dont know if its possible, but: would it be possible to implement a certain sponsor balance (like for teams, but for sponsors) - and the sponsor itself gets simulated, maybe also to real life comapnies via stock market shares (of course just the real numbers that get updated every day, but no real company names). So if a reallife company does bad, the sponsor does bad and if its in the red it cant pay teams - or cancelles (for a fee) the sponsorship all by themselfes. Sponsors would need some income in return - that could be linked to the teams success, which connects to the next point:
*min requirement: we already have the min races clause. thats a great start. additionally (when the commercial negotiates, see above) you can set some goals: a podium, a win, or 100 points this season or whatever. that can sweeten the deal, improve condition like income or length - but the sponsor has the right to cancel the contract without a fee if the goals are not met.
the connection to above: if the team raches the objectives, the sponsor will make money and thus not go bankrupt. (unless the stock market crashes
)