As has already been pointed out, your ISP service straight out of the modem is great and doesn't need SQM. If you still want to add a capable gateway modem, the NanoPi R5C with 2GB of memory for ~$70 shipped on amazon could be a good and easy option.
The R5C has CPU capability (quad ARM A55 efficiency cores at 2 GHz) equivalent to the much loved GL-MT6000 Flint 2, two 2.5 Gbps ports and plenty of memory and flash for whatever.
I recently tested my R5C and it seemed to top out around 450 Mbps with CAKE/layer_cake with my 570/23 ISP service. Different scripts or fq_codel may improve on that, but that works for me. If you need more CPU for faster SQM (which you currently do not need at all of course), the NanoPi R4S with a couple A72 performance cores on it would be a better option, but it does cost a little more.
A dual core Cudy WR3000 or the like running efficiency cores at only 1.3 GHz is going to be quite a bit slower. For slower ISP service or as a dumb AP, sure, but I would not be looking at one of those as an all-in-one gateway router for half Gig plus ISP service.
Unfortunately you've already acquired a collection of less than ideal hardware for OpenWrt. It happens. Most of us that tinker around with it have cast off hardware stored in our closets. Rather than remain attached to it, it might be better to rip off the band aid and start over.
You could try selling off your TP-Link all-in-one on ebay and picking up a GL-MT6000 Flint 2 instead if you want an all-in-one with a capable CPU that runs OpenWrt. That and your modem is all you would need in that case. Just a thought.