You are being mindful of some real risks and concerns.
That said, there's no reason to have to feel like you need to take a stab at it totally by yourself. Heck, that's what I'm doing in the day to day - putting together a few of these portfolios for people. There are certainly advisors and managers who know the academic side of this and can diversify away from concentration risks and such.
There are also many great tools in the relatively recent future that have opened up for common investors to diversify smartly. You can get pure trend following wrapped up in an ETF. You can get 100% equity 100% bond wrapped up in an etf at half of the size, so you can fit Gold and Trend in a portfolio with no hassle (both are low return but great from a diversification standpoint, so most retail investors want to lever up a bit to get back to the standard SP500 return/variance profile).
If you don't or can't find someone who can math it out for your individual case, it's also an option to just pull a few of the best proven individual ideas, such as "have some international diversification". ETFs are easy to grab here. Even 10-20% can pad out some of the US concentration and dollar exposure a bit. Smallcaps, like you said, are another way to shift away from the "size factor" dominated index weighted ETFs. The Nifty 50 bubble in the 60s was a major catastrophe, and history rhymes, so why not hedge a bit while still sticking to a proven investment framework? No financial advisor is going to look at an equity slice of 80% S&P, 10% international-ex-US, and 10% smallcap and say it's a stupid idea. Actually, they will probably say it's a smart improvement over 100% S&P.
When it comes to smallcaps, the caveat is that the Russel2000 is filled with fairly low quality companies. High debt, value traps, you've got it all. That's diversification, but probably not the type you are seeking. That's why smallcaps are a great target to add some factor based investing returns. This is another diversified source of return in and of itself (AQR as 150 billion under management here), but why not overlay some Quality and Value factors on top of the smallcaps? Now you have a sprinkle of factor investing edge in the portfolio alongside the smallcaps, while not drifting too far from the status quo since smallcaps are a small allocation. Not going to toss out specific investment ideas but there are ETFs/mutual funds for this! Just stick to low fees and a long track record. A question like "I want 10% in smallcaps, but I want a factor overlay for Quality and Value", is a question that would actually get money out of a good advisor by the way.
Here's a good, slightly wonky, lecture on some of the esoteric yet quite "in your face" risks within a complex dominated by passive and index weighted flows. It's not at all a misguided though to think there are some risks out there in the left tails. This was a lecture from 5 years ago: https://www.youtube.com/watch?v=x-rJciYZmi0&t=1s