Should I use 20/40MHz channel width for optimal performance?

Using narrower channel (i.e. 20MHz only) will give you slightly longer range.

But as @gotsprings wrote: use wires (and/or multiple APs).

Ok thanks for the tips. I should have bought the ax3 then... :/

As long as you want to remain within country regulatory limits, device with big ugly array of antennae won't help much with range (in principle it is only allowed to enhance reception). Even more, with high gain antennae it's important to orient them optimally ... and optimum for one station/location often isn't optimum for another station/location.

So in short: ax3 vs. ax2 wouldn't make a big difference.

It's a simple fact: radio will only work fine to certain extent (and that includes range, stability, throughput, etc.) ... some vendors may be a bit better than others, but it's the general principle (after all, they are all supposed to stick to same regulatory limits regarding EIRP). It's also simple fact that obstacles (e.g. walls of anything but thin paper) will degrade performance (range) a lot. So when reading the "propaganda leaflets" telling a big range of wireless AP ... it might be true when used in open terrain without any other devices using same piece of spectrum. When it comes to buildings in a neghbourhood, the usable range usually shrinks to single room (if walls are made of thick bricks or reinforced concrete) or perhaps to adjacent rooms (if walls are not too thick and rooms are not too big). With lower frequencies (2.4GHz) range is sightly longer (may work through one additional wall), but that means that interference (from neighburs) will be stronger as well.

So again: get some wires laid and install multiple APs, placed on trategic places ... you can use the AP you already have to do some measurements (place it somewhere, go around with a phone running some measurement app ... reposition AP so that you get at least -75dBm of signal strength in all places you'd want to use your WiFi ... the areas where your devices will be used most woudl ideally be covered with strong signal from a single AP, the rest should be at least 10dBm, ideally 20dBm, weaker). When you determine positions of APs, lay UTP cables connecting them a central (network topology wise, not geographically) location where your main router will be placed. It can be a router/AP combo, but I recommend it's a dedicated router (so you can reposition your APs, change AP vendor or anything else without having your main router affected).

As I understand lowering the antenna-gain will boost the antenna range, is there a limit for the "highest power" setting on the ax2? E.g. setting it to 0 should make the signal the strongest ?

ax2 has (almost certainly) set min-antenna-gain property to whatever you can set the lowest value. And if you haven't set antenna-gain to something higher, changing it won't make Tx power any higher. Ditto for tx-power property, by default it's not limited due to explicit property setting. So by default, there are two causes for Tx power limitations:

• country regulatory limit of EIRP
  you can try to lift that to higher level while being illegal (and you bear the consequences). You can go through list of countries and find one with most generous regulatory limits by running command /interface/wifi/radio/reg-info number=0 country=<country name> (where <country name> is country name properly capitalized, e.g. Brazil)
• chipset capabilities
  power varies according to MCS value (i.e. air interface rates) ... and you can't do anything about it, so this is the hard limit of Tx power. The table is available on product page.

set country USA

If one is located in one of ETSI countries, then setting to "United States" will prevent them to use otherwise legal channels 12 and 13 (on 2.4GHz band). The example I provided is better than US in this aspect. It does miss U-NII-4 channels, but these tend to make problems these days (not many clients support them).