CBsubs force show a field at registration depending on two plans combination

Too complicated, this all pleads for CBsubs option plugin release
To handle these scenarios

Too complicated

Should just be a matter of the HTML in the description and 1 auto action. Maybe 2 for upgrades after already being registered unless you just display the field in profile edit using CBSubs Fields Tabs Protection.

this all pleads for CBsubs option plugin release

It won't help you here. It just provides a dropdown to select additional options of which adjust the price of the plan.

It just provides a dropdown to select additional options of which adjust the price of the plan.

Yep that's exactly what we need, adjusting the price depending on different options.
This way we could have only two plans : one yearly with 5 prices and one monthly with 5 prices but with 12 months compulsory recurring
For now we have 5 yearly plans and a sub plan with fees for setting a manual schedule of 12 sepa payments for the yearly plans.

That's very hard to get a simple schedule of payments with different prices with CBsubs

In that case I don't really have a timeframe for CBSubs Options, sorry. It still needs a lot of work before it can be released and likely improvements to CBSubs core code. I'm also not sure if it even works with recurring payments as it has never been tested with recurring payments. Ideally we'd like for it to not only be able to modify price, but possibly duration as well.