I was hoping not having to do a detailed analysis and keeping it rather generic, as it gets complicated quickly… 
On a high level overview, the more additional networking infrastructure you need (anyways), the more the power consumption of a single piece (the router itself) drowns in the overall tally - but that's not my main argument here.
Let's postulate a few basic conditions from the start:
- a Fritz!Box like all-in-one device will always win in terms of power consumption, yes it will chug 15-20 watts, but if it really remains the only networking device in use, it sticks to that usage only.
- I consider 'modem' and 'telephony' external to the power usage, as -with OpenWrt in mind- neither will be served by the OpenWrt router, so in the electricity tally it doesn't make a difference if the same modem is used in front of an OpenWrt x86_64 router or a plastic router
- yes, I'm obviously aware of the bthub5, but as you've noticed yourself, it's very borderline as an all-in-one modem-wireless-router
- yes, I'm aware of chan_lantiq, but asterisk "is not for everyone" (to put it mildly), so another solution is probably wanted anyways
- --> there isn't really a lantiq vr9 device like a bthub5 with FXS ports
- the bthub5 is relatively unique in providing good wifi (ath9k/ ath10k), most lantiq all-in-one device cheap out on this topic and go with ralink or lantiq WAVE300 (neither of which are usable)
- FXS ports mean one core gets reserved for the voice core, in other words you lose half of the performance - which is already lacking in SMP configuration
- AVM's devices aren't supported by chan_lantiq
- the 7490 isn't supported by OpenWrt (yes, I know, external development that gets it working - not merged yet, won't get FXS/ DECT working either)
- the easybox 904xDSL is its own world of pain (not officially supported, very special hardware in many different tangents)
- this is considering x86_64 for "fast" connections, let's say >=300 MBit/s (cable/ fibre), which are hard to get working on OpenWrt with 'standard' plastic routers - x86_64 obviously does not make sense for a 100/40 MBit/s VDSL contract, which can easily be taken care of by ath79, mt7621a and just about anything else.
- this also means the modem is external to the electricity tally, as ONT/ cable modem can't be done by an OpenWrt device anyways
- to quote myself, I'm considering the case of "and once you cross the border of 4+1 ethernet ports and a single AP" here, so the need of an external switch (at least one) and an additional AP (at least one)
- neither if which is an 'uncommon' scenario' for an enthusiast household, even a small(er) one
- as we are looking at faster WAN connections (>300 MBit/s), I am comparing against modern/ faster wireless routers - not an old mips getting along with 5 watts
- ipq806x (cortex a15) uses around 15 watts idle, that's kind of the ballpark I am basing this calculation on
- mvebu (cortex a9) might be better, but I'm disqualifying it here because of mwlwifi (yes, the Turris Omnia is special, but also quite expensive)
- ipq807x can be better, depending on its wifi/ wired specs
- I don't have values for mt7622bv+mt7915, but I'd guess >=10 watts
- I assume 5-11 watts for an x86_64 router here, which is a reasonable figure - a modern 4-port/ Atom based one should be close to the lower end of that range, a cheap/ used device closer to the upper end of it
- there is very little difference between a 5-port 1 GBit/s switch and an 8-port 1 GBit/s switch, nor is there a significant difference between a managed- and an unmanaged switch of the same port count, in terms of its power consumption - you do have to skip multiple sizes, to get a noticable delta
- the number of active ports matter more, than the number of ports in total, assume 1.0-1.3 watts per active (link-up) 1000BASE-T port, less for 100BASE-T, considerably more with 5GBASE-T or 10GBASE-T
- EEE helps a bit, if supported on both ends
- some devices drop to 100BASE-T on link-down
- in terms of the overall tally, it doesn't matter much if a port is on your router or your external switch
- a modern (post 2015) 1 GBit/s switch will be much more energy efficient than an older one (pre 2010, often with transformer based PSU, no EEE, early silicon)
- the number of active ports matter more, than the number of ports in total, assume 1.0-1.3 watts per active (link-up) 1000BASE-T port, less for 100BASE-T, considerably more with 5GBASE-T or 10GBASE-T
So let's look at the hardware that can do >300 MBit/s (maybe with SQM):
- x86_64
- RPi4
- NanoPi r4s/ r5s
- with sqm maybe up to 500 MBit/s, give or take
- mvebu
- mostly disqualified because of mwlwifi (yes, I'm aware of the specific QCA wifi in the Turris Omnia, but the Linksys WRT range is by far the more common device)
- I'm guessing about the sqm performance here, early benchmarks suggested 1 GBit/s, but DSA has dropped full-duplex performance and modern -non-artificial-benchmark- uses might be more demanding, what about ~600 MBit/s?
- mt7622bv
- with sqm maybe up to 500 MBit/s, give or take
- ipq806x, I wouldn't recommend it above 500 MBit/s without sqm (maybe up to 650 under ideal circumstances), not more than 180-190 MBit/s with sqm (PPPoE might drop the numbers further)
- ipq807x, this is still hampered by the 'unoptimized' switch drivers (in the absence of NSS offloading), about 600 MBit/s without sqm, I didn't measure sqm on this platform (as the main slow-down comes from the switch drivers, and the four cortex a53 cores being quite fast, there might not be than much of a dip here)
- filogic 830 isn't really here, yet (should be a good speedup relative to mt8722bv+mt7915, bringing the specs in line with ipq807x)
So roughly speaking, below 500 MBit/s you may get away with one of these plastic wireless routers (mt7622bv+mt7915 or ipq807x), above that you're looking at x86_64, RPi4(, maybe NanoPi r4s/ r5s).
Now, what is the power consumption of a RPi4 with PSU, USB3 1000BASE-T NIC and PSU?
--> my guess is around 4-5 watts plus, ergo not that much different from the 6-7 watts of modern Atom x86_64 - everything else being the same (external switch needed, external AP needed); this figures for a NanoPi r4s/ r5s or dfrobot base board plus RPi CM4 shouldn't be much different either.
If a mt7622bv/ filogic 830/ ipq807x wireless router gets you by, it might win - but not by that much.
So assuming you have a fast line and 3+ discrete networking devices (router, switch, AP) plus modem/ phone, does 6-10 watts for a x86_64 device (compared to something like a RPi4/ r4s/ r5s) really make a tangible difference in terms of median electricity needs?
Details do heavily depend on your your exact environment and requirements, for sub 200 MBit/s WAN speeds you may get by with a single plastic router - beyond that, the air is getting thinner.
For a new 6-10 watts Atom device with four 1000BASE-T you're looking at ~250 EUR from Bezos' Inc. or Jack Ma's market place, a used device like Tips for getting cheap used x86-based firewall with full Gbit NAT (a PC Engines APU) if you are in the US (sophos, gateproct, cyberoam, barracuda, etc. - baytrail-d Atom based, four 1000BASE-T ports, 30g/4096) goes for 30-100 EUR, depending on your patience and persistence.