Ok, it's been fixed. Having watched how it was done - yea I could have done it!
It was the water pump in the OWB. As soon as the technician heard the sound of my outdoor pump, he said that's what is bad. It was making a slight humming noise, and honestly I never really noticed much of a change in the sound the pump made. He said the impeller is most likely damaged. He offered to just replace the impeller to see if that does it but I said I'll buy the part and replace it myself and keep it as a backup.
My novice plumbing knowledge truly shined when I said great how do we do this? There were two valves, one on either side of the water pump (circled in blue), that he simply turned to cut off the water. He also turned the other valve on the far right. With all valves off, he loosened the 4 nuts, removed the old water pump and replaced it with a new Taco 009 water pump. Minor wiring needed to be done to take the old electric plug off the water pump and put it on the new one but just keep track of which wire goes to which when your swapping if someone else needs to do this. He also loosened the motor from the impeller and rotated the motor/electric box mount so it sits sideways, the way the new motor was mounted the electric box was facing the insulation of the OWB and wouldn't fit. He just loosened the 4 bolts on the Taco pump, pulled them out, rotated it and put them back in.
When putting the new water pump back on, he did not fully tighten all of the bolts at first. I'd say they were 90% tight, he plugged the pump in and he slowly turned on the valve below the water pump and allowed a bit of water to come out. He then slowly turned the yellow handle valve on the right hand side, waited a moment, then turned it off. He did this a few more times, and he said he was doing this to ensure water flow was not being restricted from within the house (I guess at the HX) but also to allow any air out in case any got in during the replacement. He then full tightened all the bolts, pretty much as hard as he could, and opened the valves all the way.
My pipes inside immediately felt as though I was touching the sun. My water temperature dropped pretty quickly from 210 to 170 as my fire was pretty tiny at the time.
I'm back in business!