Hi @plaguedbyfoibles 
welcome to the forum!
This is a non personal advice and public forum. The kind of questions you have are answered by IFAs regulated and permitted by the FCA to provide advice. Freetrade provides an execution only service - no advice. The community forum guidelines encourage you to do your own research on investment choices.
Diversification is a good way to manage the risks inherent in share trading. You might want to review Passive Investors - ETF Portfolio Discussion to get an understanding of what kind of considerations people put into choosing ETFs.
You might also find these useful:
(And many more at Learn about investing with Freetrade’s guides)
Good luck in your investment journey!