Design layout with a Teensy 4.1

[metertech58761]

So I have this project that's been bouncing around in the back of my head for some time now...
I finally decided to see if I can use a Teensy 4.1 module for the logic / interface side of things, but I need some guidance with selecting the pins to use on the Teensy 4.1 module.

First, power: Does the Teensy generate its own 3.3V supply or do I need to have an external 3.3V source? I would have 5V coming in from a power supply board I set up.
If I understand right, I just connect the 5V supply to pin 48 and then pull the 3.3V for three things (display, relay input pull-ups, power for the level shifter) from the Teensy?

The functions I need to allocate:

Four digital inputs (to sense relay closures)

One digital output to drive an optoisolator

Two I2C lines to drive an LCD

8 pins for a 4x4 matrix keyboard (all 16 switches in that matrix would be used)

One pin for a serial data input - this will need to be level-shifted from 5V

One pin for serial data output - this will need to be level-shifted to 5V

One pin to set send / receive mode - this will need to be level-shifted to 5V


I will also need to generate two timing waveforms, both 50% duty cycle.

One is to be a switchable 12.5kHz output, as it will be XOR'd with the serial data line (Manchester encoding)

The other is to be a continuous 2439 Hz output to time the interrupt code.

Can these be generated on-board or am I better off using a dedicated module to generate these?