In some of the photos, some of the leaves and stems seem to have a grayish, pebbly rough surface.
Could those be insects sucking the plant juices out faster than the roots can funnel water in?
You might spray with water/soap/alcohol, or just wipe the suspect spots with a Q-Tip dipped in alcohol.
I think any container holds much less water than any in-ground plant has access to. And if the container is small, fast-draining, and hot due to direct sun, that container may not be able to hold enough water from one watering to the next.
Does the plant only droop and wilt during the hot or windy part of the day? That suggests that transpiration exceeded water uptake for several hours.
>> The soil I used is an all-purpouse one....In the garden center I was told that it would be perfectly fine for it.
I hope your garden center clerks have much more integrity than mine! In six out of ten garden stores near me, the clerks know nothing but will say anything. In 7 out of ten, they will say whatever gets them a sale.
>> And I also mixed in some grit, to make it a better choice.
THAT makes me think your soil mix may be OK for rapid drainage. But does it hold enough water to get them safely from one watering to the next?
I usually heft a pot to see how much water is left. Digging up the root ball is conclusive and accurate, but hefting is fast and easy.
>> having gravel in the bottom of a pot doesn't increase drainage (it actually reduces it- the technical term is a perched water table)
I agree. The mix has to be fairly uniform from top to bottom if you want water to drain by capillary action from top to bottom and then out the bottom. Or at least it should change texture gradually.
Sudden, abrupt changes in soil particle size can prevent capillary "connection" between the layers. Once you break the capillary connection, there might as well be a 5-mil-thick plastic bag blocking off the lower part of the pot.