Let's me take on your issues one-by-one:
the current implementation has bugs
Yep, and when you transpile such a code, what makes you think those bugs will not be transpiled as well? With a rewrite, your new system will also have bugs, but other ones, and not necessarily less.
the specification (requirements, expected behaviour) may be incomplete and is distributed over around 2500 jira issues (~ 1250 requirement issues)
Neither a transpilation nor a rewrite will fix that. Btw, when porting a program to a different platform, I would not use resolved issues from an issue tracker as a replacement for a spec, I would use the existing system and it's current behaviour as a replacement for a spec.
the implementation (the code written) may already deviate from the requirement (incomplete implementation, misunderstanding) or just buggy
A transpilation will not change this. A rewrite by people who take the old system as a template will probably copy these issues.
the external team is new and took over the code from the previous external team
You told us about an external team and and internal team, but forgot to mention who has the responsibility for maintaining what. However, I don't think any of your two approaches will change here much.
the platform has restrictions regarding mass data handling (creating 15 million rows (15 000 000) during an import proves to be challenging
Maybe going to a different platform might fix that. But usually solving an isolated issue like that in an isolated way is much cheaper.
budget for the external team size is tight
internal employees (apprentices) are available but are inexperienced
internal organisational politics are playing a role as well
These are all indicators for better not changing the platform, neither by a rewrite nor by transpilation.
What I miss completely in your post, however, is any kind of reason why you want to move away from the current platform. Are there hard technical or legal reasons why you need to leave the current platform? Or do you expect to make development cheaper or quicker on a new platform? How critical is the system for your business?
Hence, my best recommendation here is: first clarify what you want to achieve (and why), then start looking for solutions - and don't be fixated on "rewrite" vs. "transpilation". Think also about organizational solutions (like training unexperienced developers), or the option of keeping the old system as it is (and solve your major issues within the bounds of the platform).
