Recommendations for modest budget

2 years ago I bought an OLED and at the time paired it with an LG SJ9 sound bar due to space constraints.  Now I have a space that could easily hold any size 5.1 system so I'm looking to upgrade.  Trying to keep the budget modest, let's say $2k (not firm).  Any recommendations on where to start searching would be greatly appreciated.  I'm not expecting top of the line equipment with this budget but I would like something that's a substantial improvement over the sound bar.  Thanks!
I don't think that the OP is going to almost triple his budget and a decent sounding 5.1 system can be had for two thousand dollars, how do I know? Because I have one! Bought the speakers and sub used and grabbed a last year model of my receiver at a big discount because it was being replaced by a newer model.  Check craigslist for sub and there always seems to be a lot of home theater speakers on there as well.

Good Luck, TISH
For your budget, you should look for all used gears, used AV receiver & used speakers and used cablings, etc. Another advantage of buying used gears is most likely they are already fully broken in in the case of electronics & speakers, and fully burned in all the way in the case of cables. most likely if the seller had been using them for a while. 

Do you need an AV receiver that supports latest 4K HDR video pass through or not? I assumed you don't care about latest Dolby Atmos & DTS-X formats since you are only doing 5.1. Did you already have 4K/bluray player? Or is it just for cable TV DVR box/satellite TV, Apple TV & other streaming movie service providers? Most bluray and/or 4K UHD bluray players usually have built-in Netflix, Vudu, Hulu, etc. 

Are you going to be using your system for listening to music as well or strictly surround sound movies (HT)? 
Here's a great system almost identical to one I put together for my brother, and it sounds absolutely fantastic for movies and surprisingly good for music too.  Includes a sub that will get you down to an honest 20Hz so you'll really feel earth-shaking explosions, etc. in your room, and the critical center speaker really delivers the goods.  This system will make your soundbar sound like a bad joke by comparison.  All of it is new with warranty, and the total cost is within your budget at $1978.  All you need to add is cables/interconnects, and @grannyring here can build you some really good Dueland wires that'll work great and won't cost too much.  Hope this helps, and best of luck in your search.

Two questions for the OP:
1) Do you mind used equipment?
2) Do you need 4K Video?

AVRs become obsolete quickly and have very little resale value.  The market for non 4 K AVR is infinite and you can probably get a great piece for virtually nothing .If you insist on 4K you can supplement it with something like Apple TV as a source and as a music streamer to boot.  A decent BRP can be had for under $100.  That would leave the bulk of your budget for a Speaker/Sub combo, or again you might be able to score big in the used market here.  The upgrade as funds allow