You cannot share pin 5, but the RPi 4B has several hardware-based i2c channels, and at least one software-based i2c (i2c-gpio), available for use. These GPIO channels are listed and defined in your local /boot/overlays/README file; there's also a copy of this README file on RPi's GitHub site. These additional channels are declared and configured in your /...
The software I2C bus settings are documented in /boot/overlays/README.
Info: Adds support for software i2c controller on gpio pins
Params: i2c_gpio_sda GPIO used for I2C data (default "23")
i2c_gpio_scl GPIO used for I2C clock (default "24&...
The only pins usable for hardware I²C are 3,5.
You should be able to also use pin 5 to reboot (but AFAIK not shutdown).
See Will pulling pin 5 low will make the pi boot up again
You can use other pins to shutdown dtoverlay=gpio-shutdown.
Frankly there is little point. Reboot only works after a soft shutdown, which leaves most of the Pi running but still ...
Slightly off topic and a little late, but a few things that may generally help when dealing with high impedance/fast changing inputs/protocols; and this device in particular, having recently been developing with the MCP23008.
As mentioned, don't let inputs float; tie them high (preferably) or low. often the pull-ups provided via software are insufficient at ...
The touchscreen is unlikely to use many GPIO. Most of the GPIO on the expansion header are likely available for your use.
See https://pinout.xyz/boards and check if your board is listed.
You can always use the free GPIO by soldering wires to the underside of the expansion header pins.