Is there a CSV import option for Bin Transfers?

No but you could do them on the Inventory Adjustment transaction, which does have an import option. Inventory Adjustments affect costing though so you'd have to be a little careful with those.

Would you recommend an IA or Inventory Transfer to different location and back?

I'd probably stick with the IA. Involving an unrelated Location seems like you'd be asking for trouble. Either way, I'd keep an eye on the costing and probably test it in Sandbox first. Just moving things between bins on the same day shouldn't affect costing, but I've run into a bunch of scenarios where costing was unexpectedly affected. There's a SuiteAnswers article that talks about costing order that you should review because with IAs, it's not as simple on the costing as which one comes first.

I think I am just going to rapid write a script to create the bin transfer. Only way forward.

sure if you have that option, it's probably safest.

Took a few hours... along with some excel magic. Moved an entire warehouse to a single bin. ✅

There is only if you have a the WMS module installed...