Congratulations on the possible gig. It is certainly in your best interests to bill by the hour, not by task completion. In that way, all the risk is on them for possible scope creep or other problems along the way. Going by task completion (for example, "firm fixed price" contracts) puts all the risk on you. Especially as an early retiree, I'd prefer to be able to walk at any time if a problem arises, without having to argue who is at fault for failure and trying to recover payment.
If you're particularly hungry for more money and don't mind the associated risk, you could negotiate a fixed price or hourly contract with an award fee, where you earn larger pay for exceeding certain benchmarks or completing tasks ahead of schedule, for example.
One other suggestion: Keep good records of your hours worked and the tasks completed in those hours, and be prompt and regular in your billing/invoicing. I.e., invoice immediately at the end of a billing period, and do it on a regular schedule. I'd say it's best to submit invoices every two weeks or every month at most. Back when I worked as a consultant, I would provide a brief summary of the work delivered along with my invoice, so the clients had an idea of what was accomplished for that invoice. They seemed to appreciate it, particularly for long durations like this gig you're considering.