If you really need to hit the $300 mark, I'd look for a Vega 56 or a 1660ti/2060.
Really depends on the settings you want to run at. Personally I think something 'super cheap' like an RX 570/580 would the best stop gap card. 1660 non-ti could be interesting. 1660ti is solid, but its value isn't as good as the 1660 (~12% increase in perf for ~20+% more money). 1660 has one of the best p/p ratios according to TPU (second only to the 570).