Read through the Bogleheads wiki, especially the pages about "lazy portfolios." There are some recommended books on that site as well.

I disagree with some of the Boglehead stuff, but the wiki is a good resource.

You don't need to do any trial and error. You just need to pick some funds and hold onto them for a long time. The funds Betterment has already picked for you are probably pretty good.

Agree with the sibling comment -- you can achieve a good portfolio allocation with 3-4 ETFs. What Wealthfront and Betterment do is pick these funds for you, then charge you 0.25% year over year for the privilege of maintaining them. It's not the best deal for the investor because the work is not particularly hard -- "managing" your investments as a Boglehead would ideally involve logging into Vanguard once a year to rebalance.

(In fairness, they do some other stuff which is more value-added like TLH, which is more work to do yourself, but again, it's hard to justify the 0.25%.)

"If You Can" by William Berstein is a good, short ebook on this subject.

Hiring an independent financial adviser was one of the best decisions I've ever made. It's an occasional check-in to support decisions I make around investments with Vanguard and my 401k. Finding an adviser is the hard part, so it's worth talking to a bunch of people so you can find someone that's comfortable with what you want to do.

I got the Wealthfront pitch when I started with my employer, but I feel much better with my current arrangement. Your comment "I have very little choice but to stay put" is never nice to hear in any context, so I hope you can move along from that place.

You must ensure that your financial planner or adviser is a fiduciary. Otherwise, they have no obligation to advise or act in your best interests.

http://www.cnbc.com/2015/06/16/is-your-advisor-a-fiduciary-c...