Thanks all for the responses.
One thing that I should have mentioned about my process as well is that I have been making separate transfers from my credit union to each individual Fidelity account and then purchasing shares within the individual Fidelity accounts. I have discovered that Fidelity allows trading to happen (i.e. purchasing shares) the moment these transfers take place even though cash has not settled. It does not, however, allow for unsettled cash to be transferred between accounts. In other words, it is not possible to consolidate my cash remainders and then purchase one or two more ETF shares. This is part of why I have been dealing with remainders.
Therefore, one takeaway is that it would probably help to make single transfer each month to a single Fidelity account, as opposed to multiple transfers to multiple accounts. The best way to do it will be to purchase all shares in a single account and then organize them into different labeled accounts *afterward* in order to minimize the size of my cash remainder each month.
The other take-away from peoples' responses is that, while it might not hurt to go the mutual fund route, it probably would not make a huge difference in the long run, and, in fact, it might even negatively affect my returns if I were to end up putting my money into a fund with a higher expense ratio than my ETFs.
And...while I would probably save a couple/few hundred dollars from cash drag expenses over the long haul by switching to a service like M1, Betterment, or Wealthfront, it does not seem worth the hassle because Fidelity offers so many other valuable features that will help me to make money -- e.g. 24 hour phone support, credit card, branches, etc.
I'll stick with ETFs through Fidelity for now and just try to minimize the size of the remainders each month.
Thanks all for the feedback and responses.