It doesn't fluctuate if the plant is watered properly. If the plant is grown outside, it will be wet if it rains. Nutrients will always leach from container media when the water drains out of the bottom.
Water only drains out the bottom if they are excessively watered, or in some cases left to dry out too much and the soil pulls away from the sides of the container so the water runs down the sides instead, so to remedy this you pack a new ring of soil around the container. If the excessive water is due to rain, it may be unavoidable without great effort (if more than a tiny # of plants) but is no greater penalty with a larger container.
I use mixed feeding outdoors and constant feeding under glass. The pot is too small if the plant needs water multiple times per day.
We agree on this, but to be clear the consideration how often per day during the hotter months once the plant has reached good size, and assuming normal growth rate rather than being stunted.
There's no issue if the surface of the media is dry. Container plants should always be watered to container capacity because dry spots can form within the pot. Proper media for the plant can not be too 'soggy'.
Roots need oxygen and pathogens thrive without it. Keeping the poorly aerated media wet reduces the air available to plant roots.
These two statements seem contradictory. On the one hand you're stating container plaints should always be watered to container capacity, which by the way is not true at all. You only want to put back what the plant and evaporation takes out. Dry spots are minimally different in moisture level in good soil as the soil wicks the moisture and further, the plant has grown its roots where moisture is present. I water when the plants look like they need it, and only enough for them to go a few days, then 3, then 2, until it gets hotter and they get larger, then only enough for them to go one day before watered again.
The contradiction is after stating to water to container capacity, then you are decreasing the oxygen penetration and promoting fungal growth, and then also gnats. Very few types of plants thrive with this excess soil moisture.
Sand doesn't increase drainage, it just holds less water than many organic substrates. Small particles of sand clog media pore space and reduce soil aeration.
Holds less water is the same thing as increasing drainage because that is the result, less water retention for situations where overwatering cannot be avoided such as excessive spring rain, or during the learning process of how much (and how little) water they need for a given size, pot size, and climate.
You mentioned earlier that gravel increases drainage, but gravel doesn't increase drainage. It makes the volume of media smaller in the pot (basically decreases the size of the pot). The gravel also pushes the perched water table closer to the stem of the plant which is the layer of media that remains fully saturated and devoid of needed oxygen.
Gravel does increase drainage, because it creates gaps that do not wick nearly as much back upwards in the pot, as soil does, allows the water to drain instead of pooling in the bottom as much, which is the opposite of pushing water table closer to the stem. It simply is not as you are describing, and does not require enough gravel to displace a significant % of soil.
That's a great size for a fully grown plant. And if you use well aerated media, and have the proper climatic conditions, planting seedlings directly into large pots will work. However, the plant posted here has damaged roots so moisture needs to be controlled so it can recover. The media isn't great either.
It is very simple to reduce moisture, stop watering it for a period then water less. However if the soil is now full of fungus, it may need repotted not to change the pot size but rather to provide different soil. Practically speaking, it is too late in the season to hope for much. Even with a great recovery, it is doubtful that it would bare fruit this year if it is even a type that would instead of 2-3 years.
The plants will not be stunted if they are transplanted before the plant becomes root bound.
Transplanting itself stunts a plant even if not root bound. In loose soil, a lot of the root hairs will be torn off unless it was already bound into a ball and stunted from that. In a more practical sense, considering the full size that a blackberry will attain, it becomes quite cumbersome to keep transplanting from a few gallons of soil, to a few more, and a few more, especially when it can just start in the right sized container for supporting full growth, as soon as hardened off so it can survive outdoors.
I never have a problem doing as I've described. I only transplant in 2-3 situations.
1) Bought something at the local nursery so comes it a little 1/2 cup container, then if the expectation of last spring frost is over, gets transplanted into the biggest pot it will ever get.
2) Started from seed indoors to get a head start, again in a little, but larger 1-2 cup container. I do this less and less in recent years, makes more sense to me to just put more plants in the same available space and seed them outdoors in their life-long containers, if not in the ground.
3) Perennials that can't tolerate the freezing winters here, so will be in a container that isn't too heavy for one person to transport indoors to spend the winter there, but then as an overwintered plant, will need transplanted into a container at least triple the size when put back outdoors in spring, to support unstunted growth the next year, resulting in a container weight in excess of 75lbs (wet), too heavy to continue moving around more than a few feet. I usually cannot get more than one additional year out of overwintered plants due to this, but get an extreme yield that second year, so the yield per available space is maximized.
I have been refining my process for decades, to achieve maximum results with the least burden... It doesn't even make sense to keep putting more effort into repotting because putting too much effort into excessive watering. Avoid both.