Thank you for all your replies!
I believe I am starting to develop some understanding about this. I have still one question left, which is simply: Once a semideciduous tree has gone into 'leaf shedding mode', is it then best to let it leaf out again in an exposed and sunny spot, or in a protected place with diffuse light?
I'm asking this, because I have lost plants while doing both things last year. Some I put in direct sun, once they were bear and they never seemed to recover, others leafed out again first in a protected spot and then defoliated again when I moved them into a sunnier spot later, and that did them in for good.
I only have experience with real deciduous trees like figs, and they love to be put in full sun, once the winter is over, but before they leaf out.
I did underestimate the need for these semideciduous plants to harden off. And so I will pay much more attention to that this year.
My citrus, Ugni's, Strawberry Guava's and Myrtle, which have been under the balcony during winter, love to be moved to the sunny spot in early spring. No leaf burn or discoloration. Just happy green plants and new buds swelling. But these can be considered evergreens. The deciduous figs also love to be put in full sun after winter, so that they leaf out with solid, sturdy leaves.
As for the 'semideciduous ones: After close inspection and reading your remarks, I see that my Pitanga's, Campomanesia's, Jaboticaba's and even my (evergreen) Feijoa's, all of which also spent the winter outside under the balcony, do get sunburn when moved back to the warmer sunny spot.
And that can make the difference of them going into 'defoliation mode' or not.
I guess what is confusing about this is that the different species seem to have different triggers for going into 'defoliation mode'. Some of them, Eugenia Florida, Backhousia and Grumichama loose their leaves in autumn, or midwinter in the garage. And these seem to want to leaf out too soon, in the window still inside, which makes the new leaves really tender and fragile.
To be clear, I do have lights in the garage - it is more of a workspace, and it has one entire glass wall. The temperature in the garage is also higher then ina 'normal garage', since I work there. Light conditions are not oprtimal for plants, but they do get enough light and maybe even a little too much warmth in winter.
At the end of the winter they also get two to three hours of diffuse sunlight in the window, and some of them start to grow very tender growth again... That growth is too tender though, and one of the reasons why I want to move them outside as soon as possible. So that they can leaf out with more sturdy leaves.
I guess the UV factor in hardening off is more important than the temperature factor after winter. I'll pay more attention to that, but for this year that leaves me with the question I stated: Is it better to give a defoliated small tree full sun, so that the leaves that come out are hardened off, or is it better to keep these in a protected spot, let them leaf out and then slowly harden them off?
I guess I may eventually answer this question myself; I have split the defoliated plants in two groups and will let you know the results in a couple of months.
But in the mean time: all thoughts and experiences are very welcome,