I 100% know what you're dealing with. I also purchased a Ponderosa Lemon tree that had the www.mycitrustree.com tag on it, from I think Rona (from the garden centre of a big box building store), Ottawa Ontario...

It was also my first citrus plant and I have done an abundance of research to find out what's wrong. I got the same moisture metre too, as one of the first things that could be wrong when leaves drop is over or under watering; don't always believe the metre, do the two knuckle test, put your finger into the soil and if it's dry that far down water it. Also, water metres shouldn't be left in the soil, be wiped off between uses and temperature has been effecting the readings on mine.

Water should flow freely out of the bottom as citrus loves well draining soil, root rot is a problem if the bottom of the pot does not have enough drainage and things get soppy. Root rot will cause the leaf drop you're experiencing but is not the only reason leaves drop, you also seem to have the little breathing hole too so this might not be your issue.

Overwatering can cause the soil to lose nutrients and that can cause deficiencies that will make leaves yellow along the vein and drop. Using a fertilizer, but not enough or of a type that will burn the slightly exposed root ball, would take care of deficiencies. (I don't see your root ball, lemon trees like to have a bit of root exposed just along the surface)

Let's talk inside/outside plants and little bugs. Outdoors is a great place to grow; Every plant should have a summer timeshare pass. Outside is also where your plant gets bugs, that's usually okay because bigger bugs eat the little ones and your plant is in balance. When you bring the plant inside the little bugs, magnifying glass sized, don't have predators.

Your 2 likely culprits, which I inherited when I bought the plant not knowing better, are scale and spider mites:

Spider Mites

Take your plant to the shower. Give the soil a plastic bag bib, put 1:3 (not stronger) dish soap to water in a spray bottle on an aggressive mist and make sure you get under the leaves and behind the ears. When your done, hopefully you have a removable shower head, turn the shower on and rinse him off.

The mites might hide under the bags on top of the soil, I don't know how to kill them all yet but this controls the populous. I did this twice (once a week for 2 weeks) when my Christmas holidays return saw a couple dozen leaves on the floor. I've done it once since when a few leaves discoloured within 1 week.

Scale

Sooty mold or greasy mold... Let me ask a few more questions, that if you answer yes to you have scale:

Have you seen some beautiful lil' dew drops on your flower buds? (Honey Dew mealy bug (adult scale) secretion )

Have you seen anything like the caramel coloured dots, on the thicker green stem, like in the following picture? (Scale)

Sooty/greasy mold can look like dry hard water drops and is the excrement of scale, a symptom. I didn't know these existed, until I bought this plant and it came with these problems, but they are extremely common.

There are products you can buy that should get rid of these but I haven't tried any yet. I'm thinking of using Neem Oil, I hear good things but the only place I can find it is online. The premise is that the plant becomes slick new bugs can't latch and old bugs get smothered. You can cook/smother your plant if you over use it though. Neem oil is all natural, safe in the soil, okay to spray on the fruit and eat, and should be able to rid you of both of these pests.

You can use a little knife/tweezers/a soft toothbrush to 'manually extract' scale but it can be onerous if you have a lot of issues or a big plant. You can soak your tools in soap/rubbing alcohol for the added bonus of killing the scale/eggs where they latch even if you can't get them entirely off.

Longest common ever but is full of months of research, good luck and all the best.

~Cheryl