Just in case anyone stumbles upon this. I went in to get a service B and have the car checked out some more since warmer weather was upon us and I was still skeptical on the noise. The technician pointed out in a report that the rear flex disc was cracked. Getting it replaced now but all the symptoms check out with the aggressive thumps/feelings when accelerating to around 20 (i heard this gets worse as the disc cracks more at higher speeds), stopping, etc.
Really annoying getting this pinned down since the noise disappeared whenever I would drive it for a couple hours at higher speeds and only come back after a week or two driving it at city speeds.