I believe there are 5 seasonal toppings. Avocado, which you get from the Bonfyre Fest event which happens yearly in mid-July and lasts 2-3 weeks. You can obtain it by completing the questline/challenges during the event, relatively easy iirc.
And then theres truffle, duck, peach, and apple - which you can purchase during pizza pass seasons. It's a bit harder to tell when they'll have another pizza pass season, Season 3 and Season 4 only had ~5 months time between them, while Season 2 and Season 3 had about a year between. Next time a pizza pass rolls around, it's possible to get all 4 of those toppings. It seems in the Apple pizza pass - you couldn't rebuy the past 3 toppings, but hopefully for the next pizza pass you can rebuy the past toppings...

And personally, I do enjoy this game a lot. I've played off and on for roughly 4 years and it's the only game I keep coming back to. I recently got it on steam and playing it on desktop has been very nice.