Slow "Real World" upload speeds despite good speedtest.net results

I am a Cox Ultimate subscriber in San Diego, CA, and for months (if not years) I have been struggling with slow upload speeds.  When testing with speedtest.net and Cox's own speed test, I am receiving 250Mbps+ download speeds and 30+ upload readings.  "Real World" download speed is as advertised, but my upload speed is nowhere near what it should be.  Here's what I have tested with uploading so far (all via hard wired gigabit computer):

• Uploaded a 4K video to Youtube
• Uploaded same video file to Vimeo
• Uploaded file via Synology Diskstation to my office (San Diego Cox Optical Internet 200Mbps)
• Uploaded file to my parents (Comcast San Francisco Bay Area)

In all of these 4 examples, not once did I exceed 800KBps (3.6Mbps) upload speed, as monitored both through my browser and my ASUS 802.11ac router.  I can understand not ever reaching your theoretical maximum, but this was 1/10th of the speeds that I should be receiving.

Here is what I have already done to try to fix my issue:

• Had a Cox technician come out to my house to verify all signal levels.  He detected a potential issue, so he ran a brand new heavy duty line from the street into my garage, he replaced all splitters/cables, and he ran a brand new line from my garage to the cable modem.  He tested all signal levels and they all tested perfect.
• I bought a brand new top of the line Arris SB6190 cable modem.
• Tried brand new Netgear Nighthawk X6 router
• Flashed my router (ASUS RT-AC68U) to factory settings.  Replaced all cables with brand new CAT5e cables.
• Connected the cable modem directly to the cable run from the street into my garage without any splitters.  Plugged in a laptop via gigabit ethernet and ran upload test.