Before you spend more time and money on a Raspberry Pi-based approach, what is the bandwidth you get from your ISP?
A Raspberry Pi is very limited in its USB and Ethernet throughput by its hardware design. You will be lucky to get more than a couple hundred kbps through it as a router. The 3B+ using a GigE chip doesn't change the fact that the bottleneck is how it is connected to the SoC.
If you have more than, say, 100 Mbps service, I'd strongly recommend an inexpensive router over tossing cash at the dongle-Raspberry Pi approach. Competent, current routers can often be had for US$20 or less for single-band and not much more than that for dual-band, such as those discussed in the long thread
The ASIX AX88179 chipset seems to be well supported by Linux-based operating systems. They are available is a slew of USB 3.0 GigE dongles, typically US$12-15 from one of the recognized-name manufacturers.