What is last good Atheros chip, working without non-free stuff?

As far as I know, chips such as AR913x are working well without need for any non-free components, however newer atheros chips, which are called qca9*** instead of AR*** do not work without binary blobs anymore.

So, my question is what is the latest/greatest chip of this family which still works well with free drivers without binary firmware blobs?

That would be the "Peacock" generation (QCA9580, QCA9582, QCA9590, QCA9592, respectively their pre-rebranding AR95[89][02] variants), limited to 802.11n - which I would not recommend to buy anymore, despite the firmware requirements for (all) newer chipsets.

which I would not recommend to buy anymore

Why not? I think having no non-free firmware is more important than having 5GHz band. 100 or even 54 Mbit/s is enough for most use-cases.

I would use LibreCMC, but I think that probably regular OpenWrt is better, since it won't pack non-free firmware into an image for device which doesn't need it anyway, but it's better maintained.

Which one of aforementioned chips is the fastest (as in CPU performance)? Are there common routers with one of those chips and ≥16M SPI flash and ≥128M of RAM?

You do realize that everything has proprietary code in it, don’t you? The only difference is if it is field-replacable or not. Even your beloved ath9k chips have more code in them than most mainframes had. You sure as hell aren’t getting raw I/Q off the ADCs. Yep, all proprietary.

802.11ac is a larger difference to 802.11n, than 802.11n[0] was to 802.11g, both in terms of effective throughput and range/ throughput over range. This also results in better bandwidth utilization, taking exclusive ownership over the frequency for less time.

The aforementioned chipsets are PCIe wireless chipsets, not SOCs - accordingly there is no CPU performance involved. In terms of SOCs, this would technically correspond to the "Honey Bee" (QCA9531, QCA9533), "Scorpion" (QCA9550, QCA9556, QCA9557, QCA9558) or "Dragonfly" (QCA9561, QCA9563) chipsets - however (aside from low-end single-band devices), these rarely come alone, without a corresponding (ath10k) 5 GHz 802.11ac radio. If that's to be avoided, you'll need to go older, probably down to "Wasp" (AR9344, usually combined with AR9580 "Peacock" 5 GHz radios). The TP-Link TL-WDR3600/ TL-WDR4300 (8/128) would be one specimen of this generation.

However I would not agree that this would be money well spent in 2019, on the brink to 2020 and 802.11ax. MIPS based Wasp or Scorpion are no match to the ARMv7 based Dakota (ipq4018/ ipq4019) chipsets.

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[0] Not the least because early draft-n silicon (e.g. "Howl", AR913x) was both seriously underpowered and suffered from quite a few nasty silicon bugs. This was less of a problem beyond the failed QCA9880-AR1A for 802.11ac devices.

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MIPS based Wasp or Scorpion are no match to the ARMv7 based Dakota (ipq4018/ ipq4019) chipsets.

ARM has its own problems, such as ARM TrustZone, which is equivalent to Intel ME or AMD PSP in its nastiness.

What about trust zone and boot lock on this chipset? Can user replace the bootloader to one which does not use TZ, or puts something compiled from source in it?

How about non-free firmware situation?

I think you got hat point wrong... It's more like you have a higher privileged firmware code running on the same CPU like (U)EFI on x86.
I don't think that TZ is very often used on consumer router devices that uses qualcomm's ipq chipsets.

It's more like you have a higher privileged firmware code running on the same CPU like (U)EFI on x86.

Either way, quite nasty, at least as far as I know, MIPS does not have this. Also PSP is based on it, as far as I remember.

I don't think that TZ is very often used on consumer router devices that uses qualcomm's ipq chipsets.

Well, what bootloader does it use? Are complete bootloader sources available, including first stage?