[LinuxAmbulance]

I love bonsai, but it'd be nice if one could start a bonsai tree that looks like a mature tree in miniature without spending decades on the process.

Is there anything that is fast growing, or is the only option buying a multi-decade old one?

[is_true]

Go explore your nearest rock formation. It doesn't have to be tall, in my experience the opposite is true. You have to chose the right time to get the tree out, soil moist level and the status of the tree itself are the top priorities, the latter depends on the season and the tree type.