Should I go ahead and read all the books to my pet and put the beanbags/plushies into my collections or is it better to sell now and buy everything again later? Or should I buy an extra of everything now while it's brand new and sell it later down the line?

I suggest that you put the plushies and beanbags in your collection for sure. As for the foods and books and toys, unless you're going for an achievement, it's best to hold onto them as an investment, since they're one-off event items. Same with the minions if you aren't going to give any of them to your pets. ^_^

Thank you! I'm still trying to get the hang of what works in Subeta (I've been playing for about a month now) so I really appreciate your guidance.

the prices of any of these are unlikely to drop very much, and the consumables will increase in value by a lot. sP are easy to come by, so it may not be worth saving them for investment purposes.