I've been in your situation until mid 2017 (actually worse, less than ~half of your speeds with no hope for improvement ('ever') in sight - fortunately without a particular need to lag reduction, until suddenly and very unexpectedly the situation changed radically (VDSL2, FTTH maybe possible in the near future); so do plan ahead even if the future looks dire). Very personally, and without a particular regard for chasing the lowest possible lag, I do put more emphasis on preparing for the future and the longevity of my 'investments'. That means I regard dual-band support (802.11ac or better) and an expected life span of ~5 years (plus another ~5 years as cold spare/ good enough for experiments, etc.) as pretty important, meaning that I tend towards overspending slightly to get the most out of my devices for the forseable future (at least 3+ years).
From this point of view, both of your options are on the low end, with little margin upwards and with little future. Very personally, I'd rate the 50-75 EUR/ USD range with ipq40xx as the lowest 'sensible' entry level class of devices (ARMv7, multi-core, 32/256 MB, good 802.11ac radios), obviously less on the used market. Sadly when I had to take this decision in 2017 myself, ipq40xx didn't exist as a supported platform in OpenWrt yet, so I had to overspend quite a bit more beyond that, more than I would have liked (which helps me now, with a prospect of FTTH).
But your options and projected budgets here are different, you are asking for advice between 16/128 MB mt7628 @580 MHz and 16+/128 MB QCA9531 (ath79) @650 MHz, both 2.4 GHz single-band WLAN, both not really runners by today's standards, but both probably being able to cope with your needs and small && cheap.
For the mt7628 solution speaks that you will find better routing and NAT acceleration for this SOC (which however is largely immaterial once SQM enters the picture, as this precludes offloading to a large extent), against it speaks the relatively low CPU performance and pretty spotty reliability of its WLAN chipset.
Regarding the QCA9531 solution you have less well tuned NAT/ routing drivers (no hardware offloading capabilities), but as mentioned before, this doesn't matter all that much once you enable SQM, as this will always need to take the long way home through the CPU and under its close supervision. Here you probably will profit from its (comparable, but-) slightly higher CPU performance. In terms of WLAN reliability, the QCA9531 chipset driven by ath9k is a clear winner.
So while I personally wouldn't consider either of these to be an option myself (not even at your WAN speeds), both should fill the bill for your stated current requirements and provide the necessary computing power for SQM to reduce your lag and get the most out of your WAN connection. Personally I would favour ath79 over mt7628, both because of the slightly higher CPU speed and the much more reliable WLAN support, but if mt7621 were an option on the ballot, I'd be harder pressed to come to a conclusion.
Decisions are difficult and always depend on your personal requirements.