Kind of like dealing with pet food allergies (I sell pet supplies).

Restrict to the basics, see what changes. Branch out after things stabilize, introducing things back to the diet (*plant*) slowly enough that if something triggers an adverse response you can catch it.

With pets it's normally a grain or protein issue.

Plants are just as tricky.

Sure I'm missing some but I check for moisture and lighting and drainage first.

Humidity and ventilation.

Long term would be soil mix, the age and condition of it, plans for the plant, size and kind of pot.
Pamper and prioritize, or write off?

I tend to question my approach and what hasn't worked and see what I can change, in a manner I can see the change help/hurt instead of trying 10 different things at once and not knowing which are good or bad..

Interesting subject!
I like to grow tropical Orchid species and my usual approach was/still is to look where the plant grows in its native habitat. I would then follow the daily weather report/ pick a town nearby and place it amongst the cities in my weather.com or similar reports. I could see when dry season or seasonal floods occurred and water accordingly. Temperature swings can be followed along with sunshine.

After the most obvious- change location, watering, any pests?.. then eventually I may pull it out of the pot. I have had a few surprises that way. You can fool yourself about whether you are watering well enough.

I find struggles in 3 areas, beyond obvious things like too much/little light, or let it get too cold.
1 - the pot is too small and roots have formed a strangling, suffocating root pie at the bottom of the pot. Woody entities are usually the plants that do this. Easy to fix, chop that off, remove old soil, then decide if it needs a bigger pot or back in the same pot with new soil.
2 - the pot isn't too small per se, but it's drying more often than I'm watering. Easy to fix - bigger pot.
3 - plant doesn't like the soil, it's pale or yellow, or leaves are dying, not growing. Easy to fix, change the soil.

I put a lot of struggling plants & happy plants in the ground vs. having so many pots to water when it's 95° every day. 98% of summer-grounded plants end up looking picture-perfect by the time frost threatens. Then pop 'em back up and try to have a better winter with the ones that struggled last time.

Two of my ah ha moments have to do with pots.
A traditional terra cotta with a Thanksgiving cactus... just too dry even though the soil surface looked moist.
And a couple of the pretty ceramic with the atta had saucer.. those clog too easily and turn into a swamp. Very deceptive. I use those only as outer pots now.
I noticed Home depot selling a few sizes of nursery pots now, even they are getting on the plan now of using thin nursery pots inside of pretty ones.