"But putting it ALL in a U.S. tracker at this point is not good advice for diversification reasons alone."
The LGUG tracker contains shares in 473 separate companies across all sectors of the American economy - tech, aerospace, commodities, retail, manufacturing, telecoms, etc. I think it's disingenuous to claim that it lacks diversity. It is certainly tech heavy, but that's because tech companies have become the most valuable.
Valuations are currently high, but monthly contributions over 15 years will iron out any bumps in the road.
Once the OP has accumulated some capital, and gained more experience, they can think about reallocating capital or contributions to more esoteric funds.
"I'd (a) use Trustnet to find out which fund managers are consistently top quartile over the last three and five years and (b) look at sectors where active management often outperforms."
If you'd done that 3 years ago, you'd have bought Baillie Gifford funds such as Scottish Mortgage, Pacific Horizon, Shin Nippon and Edinburgh Worldwide - and you'd have lost a huge amount of money vs massive growth for the North American markets.