In general in Belgium if you work overtime that time is then alloted to "inhaalrust" up to a certain point (you're never able to work more then 11h/day or 50h/week). Only if you have like a specific agreement where you work "voluntary" overtime, which is not counted towards inhaalrust, but is the standard 50% extra or 100% extra on sundays/holidays.
As for unions, normally if a company reaches 50 employees they are required by law to have a union rep. How exactly that works I don't know. Also while I personally also have never heard of a software specific union, you can always just go with the big normal unions if you want, it's not like they're going to turn you away.
More info can basically be found on https://werk.belgie.be/nl/themas/werkbaar-en-wendbaar-werk/a... (also available in French).
As for unions, normally if a company reaches 50 employees they are required by law to have a union rep. How exactly that works I don't know. Also while I personally also have never heard of a software specific union, you can always just go with the big normal unions if you want, it's not like they're going to turn you away.