Hello everyone! I'm trying to set the best frequency for my home router. Can someone tell me, what should I be looking at here and which frequency is the best to pick? Thanks in advance!

Depends on a couple of factors:
- what can your client devices use ? Some might not be able to use the higher frequencies (>5754 or even >5600). SO you may already be limited to using frequencies below 5600 (as most of your environment seems to be doing).

- what channel width do you want to use
For 20Mhz, every channel having 0 load should be able to be used.
For 40Mhz, you're already at 5660 since that's that first one with 2 consecutive 20MHz ranges free.
And so on...

Avoid 5300, 5200 and 5500 if possible.
You can use them since signal level is rather low but it will cause interference for your network making things slower.

5745 or 5765 are unused and not in DFS channels range (in most countries) to avoid possible radar interference. All widths are available on these frequencies. See more in table on Wiki page.

I tend to avoid 5745 and above in public settings as some older client devices are unable to use them.

Depends on a couple of factors:
- what can your client devices use ? Some might not be able to use the higher frequencies (>5754 or even >5600). SO you may already be limited to using frequencies below 5600 (as most of your environment seems to be doing).

Let's say I would like to use different devices (old and new). I don't want to be limited with this. What about primary and secondary, should I pick only primary frequency or it doesn't matter which one should I pick?

- what channel width do you want to use
For 20Mhz, every channel having 0 load should be able to be used.
For 40Mhz, you're already at 5660 since that's that first one with 2 consecutive 20MHz ranges free.
And so on...

I didn't get this part, could you please give me more details?

that is something new in v7, i wasn't aware of that. cool

Let's say I would like to use different devices (old and new). I don't want to be limited with this. What about primary and secondary, should I pick only primary frequency or it doesn't matter which one should I pick?

You never specified your device but most only have 1 radio per band. So 1 frequency.
Secondary frequency is only used for a split 80+80MHz channel which as far as I know, none of the normal AX devices can use (no 160Mhz, 80MHz only). The newest Routerboard models might be able to use it though, no experience there.

- what channel width do you want to use
For 20Mhz, every channel having 0 load should be able to be used.
For 40Mhz, you're already at 5660 since that's that first one with 2 consecutive 20MHz ranges free.
And so on...

I didn't get this part, could you please give me more details?

You need to specify which channel width you want to use.
Smaller channel usually means more stability but lower speed.
But if you want to go wider, you need to use adjacent frequencies so the wider you go, the less options you have to find a block which is not being used already by other stuff.

I added 5220-5280. I hope this will work correctly. For now it seems stable.