No personal experience with it, but a little searching turned up M1 Finance. They've been around for 10 years and I see lots of search results for them, so there's at least some legitimacy.

You set up what they call a "pie" made up of "slices". It's a pie chart of your stock and fund choices. You can buy partial shares and it looked like you can buy both funds and individual stocks. No idea if there are minimums, I saw some comments saying they don't have every fund, limited or no bonds, etc, etc.

But you set up your pie, set up automatic investment (you can also do manual or fund just a single slice in a transaction, or various other options), and then it automatically distributes across your pie to the percentages you indicate.

Depending how many slices you plan to set up, it may take a while, but it seems to be a way to do what you want if you're willing to determine all the specific things you want.