Notwithstanding your last para which really can not be disputed, your post would seem to remove the ECJ from the list of competent tribunals for this case.
The commission’s letter is the start of a lengthy process that could end up in the European court of justice. The EU court in Luxembourg could impose huge daily fines for continued breaches.
UK put on formal notice over internal market bill, which ministers admit breaks international law
I don't think the EU has anywhere else it could actually go.
However, I did find this
Article 86(1) of the WA makes it clear that any cases pending before the CJEU at the end of the transition period will fall within the CJEU’s jurisdiction until they are finalised. This includes decisions on appeals.
Article 87 of the WA adds to this more specifically. It states that the European Commission has four years from the end of the transition period to bring infringement proceedings against the UK for breaches of EU law that took place during the transition period.
The Court of Justice of the EU (CJEU) interprets EU law. Although the UK has left the EU the CJEU will continue to have jurisdiction over some matters affecting the UK during the transition period, and beyond.
I'm still trying to work out if the WA is null & void if no deal transpires and the UK reverts to WTO.