But Pro Micro 16MHz is 5V, and Wacom TTL tablets use 3.3V. Am I missing something? or do i need a separate 3.3v power supply?
You need 3.3V supply (ie. simply linear voltage regulator known as LDO, that can change 5V to 3.3V) for Wacom.
Next, You have two options:
1. Cut right trace on pro micro board. Goal: voltage from USB go to RAW pin bot not to VCC pin, while VCC is still connected with ATmega32U4 supply. If You do it right, You can connect LDO input to RAW pin and output to tablet VCC and pro micro VCC.
2. Pro micro run at 5V (USB), tablet at 3.3V (LDO). Connect tablet TXD to pro micro RXD via level converter or just wire. Connect pro micro TXD to tablet RXD via level converter or just resistor within range 4k - 10k.
BTW Mine tablet works fine without that resistor because I fund 10k resistor on board. I'll never offer this solution for anybody because somebody can destroy tablet this way.
Also, since bernard has changed the Waxbee software to fit on other places, does that mean I don't need a USBasp device to upload the sketch?
Waxbee software (PC) will generate hex file. It's not an arduino sketch. You cannot upload it via arduino software. You need to use avrdude and command-line interface.
I should repair broken bootloader, test this way and post code for do it without using USBasp.