Definitely agreeing with alice and ES on their comments there.

There really appears to be rot among there, like stem rot.

JJC - your pot looks quite ok there. Not sure what the condition is on that last leaf, as I can't see for sure if the region that connects the leaf to the stem is changing colour - becoming off-colour or whitish etc.

For the latest photo, the media is now looking ultra dry.

In any case .... this particular experience with the plant's health issues can be used to advantage.

Assuming you have nice growing temperature range, and nice lighting level ....... your potting method appears to be ok. And assuming your watering schedule was ok too ------ ie. not overly wet or soggy roots most of the time ..... then make sure to consider whether or not your orchid gets at least adequate (but not necessarily constant) gentle air-movement. The gentle air-movement can definitely help cut down or eliminate rot issues.

Although - if by chance your media was overly wet most of the time ----- leading to root oxygen starvation and then root rotting, just try out one method of watering. Just try it for a little while - not just on this orchid, but any orchid ------ see the attached pic ..... use a spray nozzle and spray lots of water in the band shown. Spray enough water so that the water gets well into the media, and really wets the media ---- all the way down the sides.

JJC --- if you haven't read details from these links below, then they can be possibly useful to you in the future.

Click Here and Click Here and Click Here and Click Here and Click Here

Yeah. I am assuming it may be stem rot due to such a massive loss in leaves. Hoping maybe some of the stem is still good after this.

I have some garden safe 3 fungicide that I can spray on temporarily until the garden phos comes in. Was surprised that it costs about $30 from Amazon.

So far I have just sprayed the stem with the garden safe. Will see what I can do with the garden phos.

The base of the last leaf has yellowed already. Not sure what else I can do.

Looking that last photo, I think you fought well but one doesn't always win... I doubt very much that the plant will survive. I think best consolation would be a new plant - where you can apply what you have learned, and look forward to a happier outcome.