Hello.
Farmers used to pay a tithe and the issue was clear. Nowadays, the s.p.s., mostly those who are not attached to a state nursery, do not immediately have some earnings, but they have to pay contributions immediately, which amount to 50% or more of their earnings. Many would open a p.p. but are afraid of this and all that follows (bankruptcy etc.) I have spoken to several people and they would all agree to an agreed % (e.g. 20%) of the actual earnings. The tax authorities have perfectly satisfactory data and there is no excuse that this is not possible.  It is the only way to shake off the fear of bureaucracy and, believe me, people would open s.p.'s in large numbers, thus reviving the stagnating small economy. Do not look for a solution in some new law or regulation, but here is the solution. As for the contribution to the pension fund, the pension would be proportional to the amount paid in (still better than nothing).