By definition, porcelain and similar finishes on teaware should be pretty much inert and safe.

This is the gaiwan I always recommend to people - it's clay but has a porcelain interior lining. Excellent heat retention for puerh, too.

https://www.amazon.com/Yixing-Teacup-Kungfu-Classic-Gaiwan/dp/B071RTTVJ9/

You can search for 100 ml teacups on Amazon and Etsy to have a nice matched solo set. I've seen some really beautiful inexpensive ones on Etsy recently.

I love that 60 ml YS gaiwan and would recommend everyone own a few for quick sessions, horizontal tastings, and sampling tea, but it definitely doesn't do the trick for a proper session with puerh. The heat loss is so great with the low amount of water that you can't really go as "deep" into understanding a tea. It's really great for a few steeps with just a gram or two, maybe 2.5g of tea at most (please don't put 4g in there as you'll have basically no room for water when the leaves expand).

Each way of drinking tea has pros and cons. Not saying the tiny gaiwan is bad but I wouldn't rely on it as my only vessel.