How do you keep your shohin deciduous trees shohin size?
It's my understanding that you can't just keep continually pinching off any new growth. So what do you do?
Some years do you let it grow out and then do a harsh cut back and regrow the tertiary branches? Does that lend itself to some coarse growth?
Or is it inevitable that they eventually grow out of the shohin size?
Yes to almost all the above.
Each year the tree will grow slightly bigger as shoots extend. Good management and proper pinching and pruning should keep extension to a minimum. There should be no need to allow trees to grow out 'some years' but there will be times every year when the tree looks quite overgrown.
Each year the areas that have grown too long are removed and replaced with shorter twigs growing from back along the branches/trunk in order to maintain the size/ outline. Usually done in winter when you can see the structure. This means there will be an annual expansion and refduction in the trees.
If coarse growth results from pruning it should be removed. If the tree has been well maintained it will be strong enough to start again, hopefully 2nd shoots will be smaller but if those are also too strong I would argue the tree probably has enough energy to repeat.
All trees thicken, even if it is very gradual so eventually the trunk will simply be too thick to look good as a shohin sized tree and restyle to a larger size will become necessary. Probably won't affect many of us, esp with deciduous trees but some species do continue to thicken quite rapidly even in small pots. I have ficus that require upsizing every 5-7 years.
All the above applies to all bonsai regardless of size. It is just far more noticeable with the smaller bonsai.