Audio card questions

[aqwserf]

Hi everyone !

I'm new to the forum but I'm using Moode for (quite) long and besides some small NFS issues (similar to here or here), I'm very happy with this distribution!

I want to improve my sound quality as I'm about to buy a turntable (and amplifiers and speakers) and I have a couple of questions:

1. Would you recommend a RCA or SPDIF connection between the RPi3 and the amplifier? SPDIF seems less subject to interference
   
2. If SPDIF, which HAT card would you recommand? The HiFiBerry Digi2 Pro seems interesting but I'll consider any other product
   
3. What is the purpose of the 5V power supply connector on this same card? External power? If so, why?
   
4. Would it be possible to plug my actual Waveshare 3.5' and this card on the same RPi3? Should I be concerned about possible interference since I use WiFi and Bluetooth?
   

Speaking about DAC/connection, I'm a little confused, despite my numerous readings on the internet over the past week: SPDIF carries digital data right? So technically, the Digi2 Pro is not a DAC, just another transmission mode? If so, my amplifier will do the digital/analog conversion using it's internal DAC right?

Sorry about my dummy questions but I like to understand what's going on and I am not an expert on this  (vast) subject

Thank you in advance for your knowledgeable answers!

P.S: for any purpose, the amplifier I'm considering is the Yamaha MusicCast R-N803D