HLM's collection was incredible but she was fairly stingy with loans. I hope now that the collection is in Charles's hands that we see loans on a more regular basis. It's not very practical for individual members to have substantial personal collections as jewels are generally sold off within a few generations thanks to inheritance tax, something sovereign to sovereign inheritance avoids.
I thought Sophie should have access to nicer pieces but maybe she chose the ones she wears.
As for being stingy .... I'm not sure .... there were only a few occasions a year that required a full on bling. Camilla had the QM's jewels available and wore lots of them. Kate has some from Diana and access to some other nice pieces. Anne has her own collection and there really is no-one else. Mrs California did not stick around long enough but had a wonderful tiara for her wedding which I presume would have been available to her for State dinners etc.
Bea and Eug had lovely wedding tiaras but are not going to appear at occasions that require bling. The cousins - Kents and Glousters have their own jewels.
QE11 pretty much wore the same daytime jewels for decades only mixing it up with broaches. Even for white tie occasions she didn't mix it up that much over the decades.