Hi Noz,
I like forests and group plantings and have a few thoughts from my experience with starting forests with younger trees like these.
I reckon the way that you've positioned the trees works. The thicker tree at the front and then a range of different thicknesses and spacings - with three tree slightly offset to the left - looking good.
Moving forward, be opened-minded. Some folk will say just let em' go and grow now as it is all about thickening the trunks, others might say look at creating different heights ... there are many different views and approaches; some better than others but none really right or wrong - if that makes sense.
Me - I like shaping/cut 'n' grow from the get go .... so from now until eternity - when you are trimming - think about the trees in the context of each other ... it might mean that it takes ages for things to thicken up but... in a few years, my experience suggests, that you'll have a young group that makes sense and doesn't look banged together; rather grown together and looking cool.
This does not mean you cannot change the design - indeed, I am pretty sure you will because ....for all our efforts to control them - trees are trees and they will do what they will - sometimes that will frustrate you - sometimes you will be delighted by adventitious buds right where you want them.
So go with the flow - and enjoy - whatever the direction you and the forest travel. I wrote this a few years back (see below) - I still subscribe to most of the ideas.
https://www.ausbonsai.com.au/forum/view ... ts#p263834
Tending bonsai teaches me patience.