I've made enterprise-size proposals while working as a consultant for a large service providers, but I've also made proposals with my own tiny SaaS for public companies.

Whenever I'm in the second situation, I partner up with a large service provider that I really trust, and they cover most of the typical (non-functional) enterprise requirements, and also handle things like integration etc (my SaaS uses simple webhooks & restful APIs, and they create & maintain an intermediate anti-corruption layer etc).

I realize I'm giving away a piece of the cake, but for me it's the most efficient way to get access to these enterprise level customers while avoiding most of the hassle...

This might also be highly dependent on the type of system you're offering. I offer a system of engagement, but I can imagine a system of record or a system of report require another approach.