Our solution like you point out is more rigid than having a lawyer write it, but for the majority of people having something that is accessible and free is worth it and then having services layer on top makes the most sense. It is easier to have a well written contract that you can "turn on and off" features or sections of the contract than to try to have GPT write a custom contract for you.
The idea of "legalzoom-style" businesses has always seemed like a bamboozle to me. You pay hundreds of dollars for essentially the form documents to fill in, and you don't get any of the flexibility that an actual lawyer gives you.
As another example, Northwest Registered Agent gives you your corporate form docs for free with their registered agent services.
The difference is that when Westlaw presents me with a decision point and I choose the wrong option for my client, my client sues my insurer and is made whole. (My premiums increase accordingly).
If you make the wrong choice in your choose-your-own-legal-adventure, you lose.
(For some contracts, this is probably the right approach).
Pet trusts [1]! My lawyer literally used their existence, which I find adorable, to motivate me to read my paperwork.
[1] https://www.aspca.org/pet-care/pet-planning/pet-trust-primer
Knowing someone who works in Trusts & Estates, that is terrible. I've often heard complaints about drafting by percentages of anything but straight financial assets which have an easily determined value, because that requires an appraisal(s). Yes, there are mechanisms to work it out in the end, but it is definitely better to be able to say $X to Alice, $Y to Bob and the remainder to Claire.
You have to think of not only what you want, but how the executors will need to handle it. We all love complex formulae, but we should use our ability to handle complexity to simplify things for the heirs - it's a real gift in a bad time.
I guess there's an understanding the being an executor is a best-effort role, but maybe you could specifically codify that +/-5% on the individual shares is fine, just to take off some of the burden of needing it to be perfect, particularly if there are payouts occurring at different times and therefore some NPV stuff going on.
I was still able to rewrite the result into something that more suited me, but for a service with a $150 price tag I kind of hoped it would do more.