I think its a defect from factory, not openwrt per se, since problems exist in stock firmware.
tends to overheat and freeze, require daily reboot.
bought it last year during visit to china. cost me about $20.
Used routers multiply like rabbits. Buy used on ebay. If you really only want 16/128, then a used Archer C7 can be readily had for ~$20-$25 shipped. Or upgrade to a EA6350v3 for the same price and get 128/256 and a 4 core processor, just be sure to confirm v3 (with Seller, or via picture of label underneath router). For a lot more coin (but cheap for what you get), an EA8500 can be had for around $55 shipped. Personally, I'm through buying new. Garbage can rescue is the way to go.
Looking at market trends and where most (open source) effort goes ARM64/AARCH64(ARMv8-A) is the way to go apart from x86-64. ARMv7 is still viable and relatively recent SoCs will most likely be supported for a long time but it's "only" 32-bit and most all "new" designs features 64-bit cores instead and not to forget crypto instructions etc.
I understand how you feel, such a waste to throw away perfectly good routers..
Personally, I have rescued many old ISP modems and turn them into APs for my family and friend (along side donating my own old openwrt router).
I have a bunch of them left still. ISP in my country include modems/router with their subscriptions, and then just practically throw them every ~2 years.
Sadly these aren't openwrt compatible
Demand for consumer router is small in my country, people here mainly buys wifi extenders (cost less than $8).
Due to this, there is very few who sells good pre-owned router, and its easier to get new ones.
I dont wanna spend alot of $ only to give to someone else in a few yrs time.
In fact, you can buy some used MT7621AT 16/128 32/512 routers for around £10 during your time in China.
There are also a small number of used routes using Qualcomm SOC, but the price is more expensive.
However, they are often used for CPU-intensive work throughout the year.
And I have a Qualcomm SOC router, and the 5G WIFI of one of my MT7621 routers can't work. In fact, the MT7620 router I have is working very badly, so even if they are cheap, I don't want to buy any more.
You are right, at least in the EU, you can't buy the MT7621 router for about 10 euros.
And even if you can entrust a transshipment company to deliver the courier to the EU, I am afraid that the freight will not be cheap, and it is not worth it.
Of course, it happens to be another thing in China.
Although these are real e-waste, they should be fun.