Thank you for the answers!
I am seeking mostly experience stories (ideally somebody who upgraded from ac2 to ax3 or something like that)...
Regarding RouterOS 6 vs 7 speed on hAP ac2 - with RouterOS 6 this device has default CPU frequency of 716MHz, however the same device with RouterOS 7 has "auto" CPU frequency which can increase up to 896MHz under load. So hAP ac2 actually seems to have about the same speed on both RouterOS versions (maybe even a bit better on RouterOS 7).
P.S. I already have a few hAP ac2 devices and I am considering my options regarding the diskspace on ac2 devices with RouterOS 7 - currently I have around 300 KB free space (with
routeros and
wireless packages) and I am hoping to be able to update to 7.19 or maybe 7.20, but I need to find a long term solution. I am considering to either mod my ac2 with RouterOS 7 devices with bigger flash memory (as described
here and
here) or less likely buy some other devices like hAP ax3 (I am using switch rules, so in case of upgrading to ax3 I would need to make another bridge and take a performance hit - this makes me a bit skeptical about such upgrade).