Plain old mutual funds. You can buy Vanguard funds fee-free from Vanguard in whatever dollar amount you like. Been that way for decades.
Or you can buy ETF versions of Vanguard funds. Same expenses as mutual fund versions, but the minimum purchase is 1 share. Free to trade from a Vanguard account (or something like this Schwab account, since its exchange traded).
Many of their funds also have corresponding ETFs, and I've found that most (all) have similar or lower expense ratios, and the minimum buy is a single share (of course, you can't buy fractional shares like you can with mutual funds, so that can be annoying).
[0] https://investor.vanguard.com/mutual-funds/convert-to-admira...
https://pressroom.vanguard.com/news/Press-Release-Vanguard-L...