Garden soil usually has enough nutrients and holds nutrient for plant growth.
Everything changes when plants are kept in pots. More watering in pots means that nutrients wash out of the pots much more than ordinary soil in the garden. Potting soils generally don't have any real soil in them which means that nutrients wash away much more easily. It does depend a lot on what soil mix is used in the pots and how often they get watered but, in general, pots do far better when they get regular fertilizer added. The osmocote is a good product. It works better if it is mixed in with the soil instead of just sitting o the surface so when you add more try to stir it into the top layer of soil. Different Osmocote have different lifespan - 3 month, 6 month, 12 month, etc. Just add more when the time is running out.
Succulent mix seems to be a good choice for bonsai. Many growers here use it too.
We can repot pretty much any time of the year here. It has probably been like a year since this one was repotted.
A year is not so long for bonsai so probably not needed again yet which means any problem with growth is more likely to be sun, nutrient or it just doesn't like the tropical conditions.
As for the last question, that might be an issue as there are bonsai masters here but we don't have many groups where we could communicate here, and conifer bonsais aren't common here since most of us go for more tropical plants of course so stuff like premna and ficus are most common. Also in general you can barely find any conifers here unless you go to mountainous regions. I thought conifers enjoy colder conditions since that's where we'd find them. I'm also a bit suspicious that they might be burnt a bit from the sun.
I was a bit surprised to see a juniper instead of the usual tropical species that do so well as bonsai in Philippines. I guess we all want to have something a bit different but sometimes that can cause difficulties if the plant does not like the local conditions. Sun burn is possible but most conifers have leaves that are extra hardy to sun because they come from places where it is cold but very exposed to sun and wind. It is still possible to get sunburn when trees are adjusted to shade then suddenly moved into full sun. It takes a few weeks for any leaves to adjust to changed light so best to change gradually over a few weeks.
The new tree has flat foliage pads which means either
Chamaecyparis or
Thuja. Both genera have a number of species and many selected cultivars for gardens now so it can be difficult to work out which species and which cultivar unless someone is very familiar with the group.
Someone with more experience might be able to narrow the ID down more