"1. I'm looking for the best sound quality and music server function under 500."Sigh ... let the opinion spam begin (including mine).
"2. ...use my phone, tablet and PC as a remote to control FLAC files "I'm guessing you meant "...use my phone, tablet and PC as a remote to control [the playback of] FLAC files [stored] on an external hard drive".
"3. Are there any RPi solutions that can compete with the 2i on sound quality"Is the Bluesound 2i sound quality any good? Compared to what?
Addressing your #3 first:
I have two RPi 3 B+ builds, both with DAC HAT's (Hardware Attached on Top): one a HifiBerry DAC+ Pro (TI/Burr-Brown PCM5122 x1), the other a Orchard Audio ApplePi DAC (dual TI/Burr-Brown PCM1794A's).
The ApplePi DAC is quite good. My baseline is the Asahi Kasei AK4490 ‘Velvet Sound’ DAC circuit in the Micromega M-100 integrated amp ($4500 srp).
Are you handy with PC's/Macs and Linux, and/or do you like to build cute little computer things? If yes then a RPi-based audio rig is just your kinda game. Fun and super-easy. For cost, like this:
Raspberry Pi 3 B+ $40
Apple Orchard ApplePi DAC $199 + $5 for stacking connector
HifiBerry case $16
Total $255
RPi 3 B+ and case are on Amazon, ApplePi DAC is direct from Orchard Audio. Owner Leo is top-notch for care, support and sheer passion for what he's producing. Ask him to throw in the stand-offs for the DAC board, if you don't already have some.
Bonus: set up the RPi as dual-boot with Berryboot, and install Volumio as music server/streamer/player, and OSMC with Kodi as media center including video playback (any compatible movie files on your NAS, Youtube playback via plugin, etc).
Your #2: use a UPnP/DLNA control point app to control playback from the RPi/Volumio. If your phone & tablet are Android, you got options. BubbleUPnP is the best, from my experience. You can use a web browser for remote-control of the Kodi media center (movie playback).
If you're on iOS, it's harder if not SOL due to Apple OS's walled-off approach.
Your #1: sigh...
You say you got a PC? And a hard drive with FLAC files on them? Assuming you have a LAN too, you don't need a "music server" - you already have one: Windows OS, your hard drive, and the SMB network protocol.
What more are you looking for from a "music server"?