So after the download of Vivado I can start on the musical project. A Arduino for IO, (good libraries), and a FPGA for the synth internals. It could be argued that an Arduino is not needed, but it would be fast for UI development, and super easy to interface with the LCD, pots and RFID reader.
The massive IO on the FPGA can then be used for later expansion, and the ADCs (high speed ones), can be used for audio in mixing. The Arduino ADCs are good at pots, and not really audio. In this way the Arduino becomes the LFO and controller/sequencer.
With serial UART talk between them, there is maybe enough Arduino pins to control the contrast and back light in software. An FRAM I2C 32kB for the Arduino can store local programming or UI translations. This leaves the FPGA flash for musical use without multiplexing it.