The fiscal rules are basically fine. Bring forward all the capital investment you can if the economy needs a boost, that's exempt.
I assume (though this might be a big assumption) that a big rapprochement with the EU right now would get noticed and risk EU-level tariffs. A low key deal next month on a few points, as has been planned for a while, is good enough for now.
There's been a lot of talk of joining the Customs Union, but the way it operates with Turkey is not considered a credible solution for the UK. It wouldn't be credible in Turkey if it didn't exist already.
https://ukandeu.ac.uk/explainers/explai ... any%20case.
There's the Lib Dem possibility of a different type of Customs Union, but that would be a huge thing to negotiate, and (like Single Market membership) I can't see that they'd waste their time with Farage and Kemi saying they'd tear it up.