Have you considered using a nested palace? You mentioning a ‘secondary palace’ makes me guess you did, but here’s the link anyway.
If you’re worried about your starting palace not having enough loci, perhaps you could use a linked list to remember the nested palaces? Linked lists are, at least in theory, infinitely extendable. Maybe also consider memorizing every fifth (or so) link in a memory palace as a sort of ‘backup’ so you don’t lose the entire chain if you forget one link. For example, your friends list could look like this:
Tom throws a basketball (his palace is a basketball court in your neighborhood).
The ball is caught by Jerry, who puts it between two burger buns and starts to eat it (his palace is a burger joint in your neighborhood).
Some of the sauce spills on… etc. etc.
Giordano Bruno also uses a linked list containing linked lists within themselves in ‘Thirty Seals’ (ie each link is the start of another list). He calls this ‘The Tree’. So you can link up the species, then add any number of species and subspecies afterward. He also goes one level deeper with linked lists of ‘Trees’ (fittingly called ‘The Forest’). Kind of feels appropriate to memorise tree species in a ‘forest’, don’t you think? 