This may be old news to some, but I just saw that Etrade is offering ETF auto investing in a somewhat similar fashion to M1. Some items that stood out from my cursory review of their FAQ:
I didn't see the launch date for the functionality, but they indicated it is 'new'
100 ETF's are eligible to be used, included are many popular low-ER/broad market funds from VG, shares, Schwab
One can mix & match ETF's & Mutual Funds in the same automated plan
They consolidate all orders of eligible ETF's for Tuesday execution & each person receives the same price
Beyond the above poinst, I was underwhelmed by the disclosed execution details for the ETF purchases