"Would love your thoughts on the beta" is how you collect politeness. You already have enough of that.
You need: did step three fail, and would you ship this to the next ten people.
Name the build
Version, date they were supposed to try it, the one flow — invite a guest, pay, export. If you list twelve flows they will review none.
Ask for a review or a decision
Review hunts defects. Decision is ship / hold. "Thoughts" is neither.
If they are technical, a design review is the same discipline with a spec instead of a build.
Mark what you already know is broken
If export fails in Safari, say so. Otherwise they spend the reply teaching you something you are sitting on. The useful answer is what you did not know.
They may have no assistant and no account. Email is enough. Thank them in the note you actually wrote — not in a paragraph your model drafted about how much you value their partnership.