If you only use one audio input on the EasyCap, your audio will only come out of one of the receiving station's stereo speakers. So I would suggest bringing them both out, or paralleling them inside the unit and having a single socket driving both.
S-video is probably not worth it unless you have an S-video source. I don't, so none of mine make it to the panel.
I've bought some LM338T (data here http://www.ti.com/lit/ds/symlink/lm338.pdf ) offering quite a step up in performance compared to 7805's (5 amps peak subject to overhead and current draw). The IO filter modulator already has its own (linear reg) 5 volt supply and I'm going to hard-wire my RPi3B+ as per the RPi foundations documentation +ve to PP2 and Ground to PP5 (actually 3-6). This negates to loss through the micro-USB but retains the protection the RPi was built with. I'm going to adjust the LM to give 5.2 Volts and mount it on the back panel.
OR..... Alternatively, I've hacked one of these nice little SMPSU modules (original in the background) to raise the display height above components to allow for front panel mounting, allow on-the -fly adjustment without messing about removing it from the front panel and added the suggested L&C filter components to the output, finally re-soldering the output connector block. The benefit of this is the display can show V in or V out, or for miserly portable operation, display off via two push-buttons.
I like the display option, but the purist says use a linear regulator. Does it matter for the RPi and DDS/switching boards? From what I've read, no.
I don't think any noise side-bands will be a problem, I live miles away.
Linear regulators are not great at removing high frequency ripple. You might want to put an RC filter in there before the LO regulator if you have the voltage headroom.
Got some nice mylar cone headphone speakers to try for suitable audio (when mounted in a cavity) or somewhere I have some mylar cone 4 ohms jobs.
Work in progress....
L to R Main power and indicator, RPi touchscreen and integrated pen holder, voltmeter module for Vin (battery when portable) and VRPi to check, centre off switch below to select display or off. Video and audio in for videocap, then 5 LEDs for band and tx indicators. Seems a shame to leave space
ABout the only extras I can think of ATM would be remote RPI power and activity leds, duplicates/replacements of those on the RPi PCB.