Less Is Best
I think Strong & Weak forces are fundamental forces along with E-M and Gravity. I've read that there is now a fifth force as well. I have the feeling that as understanding progresses, we'll have others too.
As such, I'd argue that a tag for Fundamental Forces be differentiated from tags for either Radiation or Radioactivity.
I agree with all your proposals
Looking at each and every tag, they all come from 2016 or earlier. They've been alive for 7 years. But even with 7 years of use, each of the tags got less than 50 questions each (and two less than 10!). It's a very strong sign they aren't attracting people's eyes and ears.
Because of that -and because there are either synonymous or more generic tags-, they can and should be merged together. They're not useful enough to help people find and categorize questions in a reasonable manner, all while cluttering the tag list.
I wish this had been asked as two questions.
I disagree with combining tags weak-force and strong-force into nuclear. We loose too much granularity with that combination. There are many questions that involve the nuclear tag and have nothing to do with either of the force tags. I also agree with @Elemtilas' analysis. The nuclear forces are basic forces and should have their own tags.
On the other hand, I have no objection at all making radioactivity a synonym of radiation. Do you not yet have sufficient rep to make synonym suggestions? At that point it gets voted on in the review queue.