FYI, if you don't mind refurbished you can get it for USD $502 from benq directly using code WFH20.
I have also been researching extensively and ended up purchasing the EX3501R yesterday. I bought it because its a good price for a 100 Hz, 1440p ultrawide from a reputable brand. The only other cost-competitive product would be from AOC, but personally I don't regard them as a quality brand.
Just looking at benchmarks I would think that the R9 390 will have a tough time handling 1440p. Even with Freesync you might dip below 48 Hz, which I think is the minimum bound for it. From benchmarks, I think you will struggle to maintain 1440p for your current games. I'm using the link below for reference:
https://www.userbenchmark.com/PCGame/FPS-Estimates-Far-Cry-5/3973/30070.0.0.0.0
Either way, I don't see how your current rig should impact your monitor purchase decision if you intend on upgrading your rig down the line.