I'd be surprised if more 'hackable' watches didn't pop up around the Sifli chips. Lilygo have an upcoming device with Sifli 52 chip. There's the SF32LB52-ULP smartwatch development board.
I've built custom firmware for a DIY OLED ESP-32 watch that is made by a few vendors before. In some ways we're emerging into that reality now but I'd admit that what Core Devices is trying to do and the general level of polish of the Pebble ecosystem is a lot further than something like what I'm describing.