We could make things deterministic by fixing the algorithms and a random seed, and/or by fully constraining every object in the diagram so randomness is not used. But that isn't using the real strengths of Penrose, so we expect it to be a secondary use case at best.

One way to think about the goal of Penrose is that you will get a result that is predictably good, and you can constrain it to get a diagram of a particular shape if you want to, or alternatively leave it open and have the solver show you 3 good diagrams...then you can pick one and tweak it further (aided by constraint solving as you tweak) to make it great. The problem with drawing-tool style systems is that you have to adjust the location of every label (for example); that's the kind of thing that should be solved for you. TikZ doesn't have a rich constraint language and solver, so in many circumstances you're left with a choice between doing lots of things manually or getting a poor-quality diagram.

Penrose will have a canonical textual format, but as it is under active development, it wouldn't make sense to say it has a stable textual format now.