My 2cent armchair answer would be ordinarily "any of the above"- I've had every single thing mentioned happen to me over the years. Infact- I just went through a similar situation myself, which was solved by a lower ball joint.
Today, however, I feel
sassy and am going to go against the grain.
It's a defective tire- cord separating.
Final answer.
A lot of times a ball joint will give you clunking/ popping.
The cord separating explains why it is getting worse.
The second time you took it to get it balanced, meese guesses... they put the tires on a different balancing machine, and it didn't work as well as the first balancing machine (or a new weight immediately fell off). That's why it immediately felt worse. Also bonus points if the offending tire was on the rear and then rotated to the front of a fwd car. As you drove it more, the cords are worsening, so when you are driving at a constant speed with vibrations coming on and off, the unbalanced portion of the tires are phasing through harmonics as the unbalanced tires spins in unison/ out of unison with themselves, causing the feeling to subside/ or worsen depending on how the tires are rolling in relation to one another.
This could be compounded with a bent rim- rim gets hit. bent, damaging tire in process. now bent rim+ separating cords. The lower the profile tire/rim, the higher the chance.
I'm probably totally wrong, but I'm betting the bank on this!
This stuff can be so frustrating.
Second guess-
dead squirrel wrapped around driveline.
Good luck.