The trick to launching a SaaS is picking the right things to _not do_. Some of the fun of a SaaS is iterating on everything from product features to pricing to support tooling. It's very difficult to get pricing, trial periods, and billing model right on the first try. So usually you're better off doing less of that, and figuring it out later.
And you really don't need a crazy sophisticated drip email setup. Doing email marketing for existing users and new signups right is something that will get you incremental gains. If you convert 5% better when you have 1000 signups per month, you'll be happy. If you convert 5% better when you have 10 signups per month you'll never even know.
Silly as it sounds, if you spend more than an hour or two thinking about sales tax before launch, you're wasting time. Screwing up sales tax at launch will not kill you. _Skipping sales tax entirely_ at launch will not kill you. In fact, very little that can go wrong will kill you. You're probably going to die because you didn't make enough go right.