My main monitor is an ASUS MG279Q (1440p144, Freesync) and GPU is an RX480.
1. Buy a cheap monitor.
2. Buy a better and bigger primary monitor.
2.1 Eventually migrate to GSync.
2.2 Wait for NVidia to eventually quit blocking use of VESA standard adaptive sync on their desktop GPUs.
Unless you want to double your pixel count to 2160p, currently-available monitors aren't much of an upgrade from what you have. How do you use your second monitor? I currently have a 1200x1920 secondary monitor in portrait mode beside my primary 2560x1440 display. They have similar size pixels. Your existing 27" monitor
has a pixel pitch of 0.233 mm (109 ppi). A 2160p monitor with the same size pixels would have a 40" diagonal. Because your existing monitor has an IPS (or VA) LCD panel, it could work satisfactorily if turned to portrait mode. (TN LCD panels are unusable when rotated.)
I suggest continuing to use your existing monitor for gaming. Purchase a secondary monitor for the things for which you use your secondary monitor.
27" 2160p60 IPS monitors with Freesync are under $300, delivered
. The pixel density is 50% higher than your current monitor. That is too small for my eyesight today, but you may be young enough to still enjoy it.
32" 2160p60 IPS monitors with Freesync are under $400, delivered
. The pixel density is 25% higher than your current monitor.
31½" 1440p60 IPS monitors with Freesync are also under $300, delivered
. The pixel density is 15% less than your existing gaming monitor.