The only real downside might be waterproofing but if you can make it work for the type C port…

I've heard that argument against having a 1/8" TRS port on smartphones before, and I don't buy it. Yes, there are lots of devices where there's just air between the connector and the rest of the electronic device, so water entering through the port can flow into the rest of the device...but there is no fundamental requirement imposed that requires devices to be designed like that. It'd be entirely-reasonable to seal it off, have the port external to the rest of the phone, have no way for water to pass from one area to the other.

There are some types of data or electrical connections for which you cannot do that, where the problem is that water's conductivity causes problems for a port itself, and the interface isn't designed to handle things being shorted, but that shouldn't be an issue for 1/8" TRS. Hell, you short its contacts just plugging the device in.