Is keep adding new variebles and casting slowly making project unstable, how many is too many?

Optimisation is a huge topic:

But, even if executed perfectly, it will not save you from inherently bad scripting / coding practises.

[…] adding new variebles […] multiple castings are toward the same actor at exactly the same time, will that cause any issues?

Should not be a problem. On the other hand, consider avoiding:

  • casting on Tick
  • unnecessary Tick logic
  • long blueprint loops
  • widget function binding
  • getting (all) actors of class
  • manual garbage collection
  • spawning and keeping a huge number of actors / widgets alive
  • spamming delays

Try to employ:

  • an event driven approach whenever possible
  • soft references
  • object pooling

The above is super-generic; no clue what you’re up to.

I started to notice some small issues that never happened before

Like what? It’s pretty impossible to advise without concrete details.