The problem with syringes is that they need to be washed thoroughly between uses and most are made to be disposable. You're going to struggle to find a syringe manufactured to be reused, and with hard wearing markings

Surely there is plastic containers that last longer than 6 months

They don't stack well and they don't do too well being tossed around in a lunch box and car

Good to know. I've never owned tupperware because it's expensive. It's always the kids that fail on most storage tubs