1) I still struggle with a bit of waswas, and sometimes in prayer, in rukoo or sujood, I’ll start by saying a dhikr, like “Subhana Rabi Al A3la…” But sometimes I get waswas on whether I said Subhana or Suphana.

So I just switch to another dhikr (like I’ll say 3 times SubhanAllah instead) without completing or correcting the first one.

Does it invalidate my prayer?

2) At the end of the salat, does it suffice in saying “As Salamou Alaykoum” only after Tashhahud?

1- No that does not invalidate your prayer. You can switch to another dhikr. Also, try to ignore these doubts and just continue to overcome such waswas

2- Yes it suffices to say that.