How does privatisation work for your companies ?

So, I know if a company has a monopoly on a market, then their good sells for +20% more.

That's a good thing, because it means more profit, more industry, more jobs, and, in the case of prestige goods, a whole load of benefits. So i always try to give my companies monopoly for their prestige goods.

However, I always end up with like 93% of market share, mostly because some industries were privatised / built by the private sector beforehand, and I cannot nationalize them for my company to buy back while i have the monopoly.

Is there a way of fixing that ? If I'm on Interventionism, and I privatise the tooling workshops I build for example, will they be bought by the private sector even if it's monopolised by my company ? Can my company buy from the private sector ?