About the trunk I'me aware of that. The only way to get the trunk ticker is to let the three grow up? I don't want the three became to big.
Indeed, you will need growth to get a bigger trunk. This does not mean your tree will stay big - often bonsai are created from bigger trees.
There are articles about how to grrow a trunk. There is an article on this website - and there are more, but I do not have the URL at the moment. Google it, or Iĺl try and post that later.
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What other three you'ld recommend to a begginer ? I really like olive threes. I also like Acer palmatum and Juniperus.
Juniper is a good choice.
Another good option would be any tree native to your area, that is suitable for bonsai.
I cannot recommend Japanese maples... I do not have good experience with them. I have quite a few, but for some reason they just don't do well - even the ones that are in full soil in my garden. They tend to suddenly get problems, even the ones that look healthy and are growing well, leaves shrivelling up, branches dying back.... but of course that could be just me (though my other trees are doing fine).