I've got strips & drops because I use cheaper meters that I don't trust 1/2 the time lol...keeps me on my toes
100 ppm is pretty damn good for tap...I'm 275 - 325 so I gave up on that for using any type of synthetics
What about a tea with calcium, ep salt, worm castings to see if there is any perk? Is it possible that amendments are depleted and all she's getting is mostly from the tap & what little was in there is done?
That big pot & soil like that...going to be hard pressed but it's always worth a try...I'd drill oodles and oodles of holes so after you do water it can dry out and get a proper wet/dry cycle which is very important for a soil grow.
Honestly that's all I got, sorry. Still a newb and always will be