Submitting an extension paper of a paper that is currently in review for a considerable amount of time? I have a paper under review for over 6 months now. Recently I have done some more work which is an extension of paper under review and wrote it as a new paper. As there is dependency on the paper under review, I have included some important theorems of paper under review in appendix of the new paper. Can I submit this new paper to (a) the same journal, although there is a paper already under review for a long time. (b) Should I submit to another journal, even before the review process of current paper still going on.
(c) should i seek advice of the EIC of the current paper under review
(d) Don't do anything and wait for the decision on the current paper under review and proceed accordingly?

Appreciate any pros and cons or suggestions/advice in this scenario.
I wouldn’t submit the follow-up paper while the first is still under revision. For the moment I’d consider the time that you’ve waited as pretty normal. So stay calm and wait some more time. If the first paper eventually gets publish, fine, then proceed with the follow-up paper. Otherwise, I would consider the option to merge both versions to a more influential single version and submit this one (maybe even in a higher-ranked journal).