1GB/month gives average throughput of 3.3 kbps ... which is something any RB device would handle. The problem, though, is that most probably usage will be much more sporadic with high peak-to-average ratio. So when deciding whether certain hardware would handle the load, one has to dimension for "average peaks", both in terms of instantaneous total bandwidth and number of active connections. I don't know if ROS has some practical limit in terms of number of established OpenVPN connections,
license level however does impose limits. hAP ac2 and hAP ax2 come with license level 4, which limits number of established OpenVPN connections to 200. hAP ax3, OTOH, comes with license level 6 which doesn't have any limit on number of OpenVPN connections. As to OpenVPN throughput (which is AFAIK not HW offloaded on any device), my estimation is that all mentioned devices (hAP ac2, hAP ax2 and hAP ax3) should be able to push a few tens of Mbps (probably quite less than 100Mbps though).