I agree with all your points, but think there are two additional key ones to consider.
First; before developing the product, talk to potential partners to understand exactly what the problem is so that you can design a solution that meets their needs.
Second; Develop a product that solves the B2B customer's problem, is totally intuitive to use, and is cost effective.
Too many SaaS products are over-complex, poorly documented, or subject to endless updates. Businesses just want a product that their staff can use without extensive or ongoing training. When I say that the solution should be cost effective, you have to look at what it makes the customer in extra revenue or cost savings, then remember that it's a marginal cost so £100 of extra revenue might only end up as £10 after wages, taxes, cost of production etc are included. When I am shopping for SaaS solutions, if the software costs more than about a third of the marginal cost, it won't be cost effective, no matter how technically good it is.