Bolting will work short term, but will kill the tree long term (a few years) The tree continues to grow. Your system needs to allow for the fasteners to be let out to not cut the sap flow.
The usual way to fasten a serious tree house to the trunk uses special hardware. Example here: https://www.nelsontreehouse.com/blog/2017/2/21/hardware-highlight-tab
Every tree house I did as a kid, or images of caches for wilderness living have used multiple trees, or a tree with heavy branches. A nailed plank on top of the branch will be pushed up by the growth of the tree. Similarly a nailed plank on the side of a tree can be pushed out (Non-ardox nail) However a few nails may not be enough to support the weight.
In your update photo, you are making a very low tree house. As a kid part of the thrill was being out of sight in the tree, with the ground a scary distance below. This is unsafe. But that was why it was fun.
If you are attaching to a single trunk at multiple points, your attachment points have to spread the load over something larger than a bolt shank, hence the large cylinders in the TAB system.
If you are attaching to multiple trunks, you need some method to allow the trees to sway out of sync. The easiest way to do this is to use something like TAB, and hang the support beams from short chains. This can make the treehouse a bit disconcerting in a wind, as platform will lift and shift in odd ways.
You also have to do this in ways that allow the stress of the weight moving about. Generally supporting a rigid object at more than 3 points at once is hard. (Think of getting all 4 legs of a table on an uneven floor.) Design the platform either to be rigid enough to be supported by any 3 of your set of support points, or design it to flex. Warning: Flexible systems can pinch.
You can compensate for a lot of this by using strong springs as part of the linkage between the tree and the platform. This makes the transition as the chains move much smoother.