We are also bootstrapped and wasted years chasing after a few large contracts like that. Sales process usually ended up taking 6-18 months, integration another 12, customer support some more, new custom feature development 80% of our dev time. By the time things were deployed half of the people we were dealing with were on to new jobs, companies were acquired, offices closed, funding rounds fell through causing them to shut down before paying us.

If it will help you get by I'd advise you to charge 4-10x more than you think is right and make them pay yearly up front, and don't count on future revenue.