Advice: Diversify my retirement investments with a Roth IRA

Large cap, mid cap, small cap, US stock, international stock, junk bond, reit.....their performance correlation are so high that you cannot reduce risk by much diversifying/allocating among them.

Decide how much you want to stay in cash/investment grade bond and allocate the remaining in any way you like.

If you are starting your Roth IRA now, your initial investment is limited and so it will not be possible to create the diversified investment that you propose. Most mutual funds have minimum initial investments of a few thousand dollars for general investments but lower amounts (typically $1000) for IRA accounts provided that you agree to make additional monthly investments via their automated investment service. 5% of $12,000 (assuming you contribute the total amount for 2019 and 2020 in one swell foop) is still less than $1000. So, your strategy may not work as you want it to right off the bat though you could achieve it in a few years' time.

Now I will wait for the brickbats and downvotes to start as people start pointing out various mutual funds that have far lower minimums for IRA accounts, totally ignoring the fact that I said "Most mutual funds".....

Welcome to the site :)

Take all this with a grain of salt, since none of us know the details of your situation, but I think that's a reasonable start. It's good that you're focusing on low-cost index funds, and looking for diversification.

Here's the things I'd personally recommend looking into as potential changes. I'll link to a lot of Ben Felix's videos, because I think he's a really solid resource to start learning the ins and outs. The Bogleheads forum is also a good resource too.

I wouldn't recommend REITs at all.

I don't think the Mid/Small blend funds are going to be as aggressive as you might think; focusing on small-value and large-value funds specifically might be more what you're looking for. Although, make sure you understand that it will have long periods of lower returns (like the past 10 years), and make sure you're prepared to commit to that for the long haul. The worst thing you can do is get scared, break from your plan, and start chasing performance. Of course, the overall market will have those periods as well.

I personally don't like the "Zero" funds, because they feel like a marketing gimmick. They pay dividends annually instead of quarterly, which will eat away some of the savings from lower expense ratios. The ratios for Fidelity's older funds like FXAIX already have very low ERs, as does similar offerings from Vanguard. Fidelity is good, but I personally trust Vanguard much, much more. They have a long history of promoting sensible investments for the average person, and the company is owned by its clients, so they're more incentivized to protect your interests.

I think 30-50% in international funds is a better long-term outlook, but lots of people will disagree with that.

If you're comfortable with the risk, then 100% equity might be a allocation than 95%. If you don't trust yourself to stick to that plan when stocks drop 25-50% during global crises, recessions, etc, though, then keeping 10%+ in bonds can help protect you against making behavioral errors in the future.

Don't let any of that discourage you, though; the most important thing to just get started, especially with automatic monthly deposits. You can always made improvements to your asset allocation over time, as you learn more.