As I understand it, everything above the mandatory minimum is considered “überobligatorisch”. It doesn’t matter whether you have more than 10% monthly contributions (your option b), an insured salary above 61k (86k-25k) or do manual buy-ins. You get “überobligatorischer Umwandlungssatz” for all of these (or possibly “umhüllender Umwandlungssatz” as is becoming more common). I’m not completely sure, though, as I don’t have a pillar 2 to confirm.
If doubling to 20% means that the employer pays 10% and you pay 10%, that sounds like a better deal than a buy-in on your own, assuming this means you get 5% extra from the employer. This may depend on the details of these options, e.g., lack of flexibility may be a downside.
Please also note that the “Umwandlungssatz” that matters is the one of your final pension fund when you retire, which may not be the same as your current pension fund (and the conditions of the current fund may change as well). The mandatory minimum may also change over the years. I.e., there are various aspects that make it impossible to predict the exact impact of certain choices.
The most important aspects while still far from retirement are the maximum monthly contributions from the employer (part of your total compensation; besides the percentage also make sure the full salary is insured) and the maximum contributions of your own money (can be increased monthly contributions or buy-ins). The interest rates / returns can also make a significant difference but that may be difficult to predict and compare.
For your last employer / pension fund, the “Umwandlungssatz” will be significant but also options for (partial) lump sum withdrawal. Depending on the latter and your personal situation, the former may be less important. Especially if you aim for early retirement, the “Umwandlungssatz” may be irrelevant.