I have spent couple hours trying your product, it's pretty damn good! Couple remarks:
1. It would have been great if I could make other variables used in Monte Carlo simulations than the market. It would allow one to model uncertainty. For example, an event could happen following a random distribution (kid arrival, move between 3 and 5 years), or a future salary could follow some normal distribution, etc.
2. The main dashboard for a plan is GREAT. UI/UX-wise, considering the number of dimensions to factor, I find it surprisingly simple to iterate over a plan.
3. Having the option to show expenses on the main plan chart would be great. Expenses are a big part of optimizing personal finances.
4. The plan comparison may deserve a different UI than the simple plan analysis. Seeing differences on the right is not completely intuitive.
5. I'm currently living in France, and we have some accounts with no amount limit but with an annual transfert limit. For example, you cannot add more than 20K€/year to the account. I believe it's currently not possible to set a cash allocation such that it maximizes transfert amount/year instead of overall amount.
6. Related to 5., but we cannot withdraw from that account without paying taxes until retrieving, then it's tax-free up to some amount per month. Maybe similar to a 401k. But it's impossible to set that up on a custom account.
7. I have set that model up, still in trial, but now I'm really scared that my browser might bug or hit refresh by habit and loose everything. The find the saving feature to be really harsh. It'd be nice to find some alternative so that it can be saved, and maybe retrieve it after subscribing? Or limit the number of parameters/events that can be added to the model? Or show in much simplified mode. In my case, I'd like to show my partner and discuss it before subscribing: I want to make sure it contains enough features needed to model our finances.
Really good job overall, I'll certainly follow your work!