So to summarize the situation a bit.
You have a AVM FRITZ!Box 4020
- on a 100/40 MBit/s line
- 750 MHz QCA9561 (single core mips 74Kc)
- 16 MB flash
- 128 MB RAM
- 2.4-GHz-only 802.11n wireless,
which you're not going to use (for the bulk of your needs) anyways, offloading the wireless side to dedicated APs - up to ~35 concurrent devices in a commercial/ hospitality setting
As you have correctly noted, apart from the 'unused' wireless side[0], these are exactly the same specifications as for the Archer c7; switching to an archer c7 wouldn't gain you anything for this setup.
Given the circumstances, I do think that these ath79 devices are a bit marginal. While they are sufficient for this WAN speed, the number of concurrent users can push it towards (or beyond-) its limits with SQM enabled (which you probably do want) or adblocking (which would be a good idea, but legally hot water when providing public access). With captive portal/ ticketing solution on top (you haven't said anything about this topic though), this would probably too much.
For a home setting, with cooperating users, this would be all fine - for a commercial setting, with 25-30 concurrent -competing- users, this might be a little stressful (both in terms of the total bandwidth and the router trying to distribute it fairly --> SQM), so I wouldn't really recommend this device (nor the archer c7, same story). While its possible, it might get a bit too tight under full load. The prices relative to their capabilities are IMHO still a bit too steep for what they can offer (in 2021, relative to the competition). Multi-core, more (CPU-) performance (at least 2*~1 GHz) and more RAM (256-512 MB) can help here (16 MB flash minimum, better >=32 MB, if you have ticketing somewhere down your todo list), especially if there's a speed bump in sight within the foreseeable future (upcoming 2-3 years). While it does not make sense to overspend in advance, having a little more headroom would probably be beneficial, especially in a commercial setting.
Just to put some examples to the table:
- low-end (not thaaat much better), mt7621a with good amount of flash/ RAM
- ipq806x, while its main benefit would be the wireless side, it can do this current WAN speed comfortably (with SQM) - economically probably not the best combination
- mvebu (a wrt1200ac/ wrt1900ac v2/ wrt1900acs/ wrt3200acm/ wrt32x might be a good fit, yes mwlwifi wireless is bad/ not recommended at all, but you don't use it for this setting anyways)
- NanoPi r2s (with active fan!)/ NanoPi r4s (not formally supported yet, so more effort on your side)
- RPi4 with USB3 ethernet card, a bit overkill - but lots of headroom.
- just for completeness' sake, x86_64 is also always an option.
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[0] not having a 5 GHz ath10k radio actually helps you for this situation, leaving more free RAM for other uses - so 'better' than the Archer c7.