There are several ways to connect these devices to Hubitat. There are several community-developed drivers and a quick search for Tasmota should reveal more than one. Since there are multiple ways of adding these devices, you will need to follow the steps in whichever direction you choose. I would personally try more than one of the methods. You may find one fits better for your situation.
Once the devices are flashed, the Pi can be shut off. Once the initial flash has been done, you should be able to update the firmware directly from the device.