I love M1! But I have come to hate their "tech". Obscure URL's (what was that stock you looked at two days ago? good luck finding it in your history!), broken watchlist, can't explore adjustments to a pie (such as two windows to submit/revert a pie change), have to contact support to pause trading, backtesting breaks if stocks are listed <5 years ago... they recently added dark mode to their app with terrible contrast of red/green (the green looks black in white mode so you have to use dark)
I feel like their UX priorities are screwed up big time. But as a platform, they're my favorite as far as concept/functionality.
You specify pies of funds with weights.
They automatically allocate funds and dividends to the underweight entries, and have 1 click for rebalancing.
It's not made for single-real time trading, there's one trading window each day.
Sure beats the spreadsheet and whole-share issues when I did it for free with Robinhood.