No, with NS Flex you get no discount on busses, only on trains. Exception is when due to works there are no trains on a certain route NS will run special busses to replace the train. . If you want free travel with regular busses you will need a subscription from your bus company.

NS FLEX all-day is also not ´free´=no pay. The same applies to the buses. It varies per area/region/province what the current offer is. I guess you will not be travelling all days in all the 20+ regions.

There was a kind of monthly all bus+tram national season, but that cost even more as for trains-so that was not sold much. A very, very long time ago it was even much reduced for those who also held a train-season, so you could have all-transport in 1.

IF you use it to explore some regions further-hen best advice is to organise it to what they offer-day by day. On the site of ROVER.nl, the dutch travellers-organisation, is a long list with all types of daily bustickets-in dutch only. Often these are reduced price in the local school-holidays

Most people use the bus to go to train-and back evening. IN such a case you have to inform at your local buscompany site.

IF you plan to take such a subscription-use it on another chipcard, very often it clashes if you also have FLEX on the same card.

Depends on what NS Flex subscription you have. With Flex Altijd Vrij you travel free always

And yet again a wrong answer, this is still sold, but indeed costs quite a lot.

https://webshop.arriva.nl/nl/webshop/product-selection/overview