"Divide and conquer", isn't a new concept.

Don't fall for it, Europe

It is spelled JOIN and it will include everything EU - absolutely everything.

Better rephrase as:
"The EU would only agree to anything substantially if it is part of a UK full Copenhagen Criteria A49 membership application".

Otherwise the TCA with at best fairly minor mitigations will be possible

What will a CU help the UK ?

The EU deals in FTAs (e.g. the TCA with the UK) and in full memberships - not in bespoke deals

More than anything "British exceptionalism" is unacceptable for the EU.
We are all plain vanilla EU citizens.

Apart from the EU will unlikely accept anything but an A49 UK application for a Copenhagen Criteria full EU membership -
how will an EU-UK custom union help your business?
It will only help for businesses with expensive Rule of Origin certifications.
The TCA has quota free, tariff free trade

Where you will surely get a huge NO.

Nothing closer than the TCA unless all Copenhagen Criteria are accepted and implemented by the UK.

Only when the full Copenhagen Criteria is accepted - and accepted forever.

The EU should reply with: "Send an A49 application for a full Copenhagen Criteria EU membership - or get lost"