How to find out to which journal(s) I could hand in my paper? During my study or even phd I had no chances to draw up a paper as a main author. Reasons do not matter here.
Now I'm already in the industry for a while and I wonder why not preparing some papers based on my phd. I have no external pressure or goal or whatever so I would only do this to simplify other's life. They can probably also figure out on their own but when somebody already did..  in principle it is already published through my phd but.. yeah, published through my university's library, so no one will ever know.
However, the work touches mainly the application of some optimization problems to make use of in a kind of an engineering field. I myself come from the (experimental) particle physics.
So, which journals do make sense?
Computer science/Mathematics not really because I barely improved the optimization problem itself. At least not on a theoretical level. Depends on the pov.

Particle physics not because it is not about particle physics. The devices are just used there. Well, they are used there very often but originally it from solid physics/electrical engineering (photodiodes) and I didn't improve the devices themselves anyway.
Electrical engineering not because I didn't take into account any circuits or similar.
My supervisors were two experimental physics professors, a maths professor as well as a computer scientist professor (who is also a mathematician). Just in case it matters.
So what's left? And/or which journals make sense?
Despite from that: Can I just contact some certain journals and ask whether it makes sense to set up a paper before I take up all the effort for it?
Considering you are talking about publishing papers based on PhD content it would be usual in most STEM fields for your doctoral supervisor(s) to be involved in this. As such, one or all of them would be worth discussing this question with as they're likely to have the best read on what / where is appropriate. At the very least they should be able to consult with or introduce you to a colleague more familiar with your specific work direction. That said it would be sensible to consider where your dissertation / lit review sources were published, as suggested in the comments, and to come up with some possible choices of your own before approaching your former supervisors