Styrofoam is down - what do we do with the ceiling?

Hello dear community,
we are currently taking over my parents' house little by little, after some clearing actions we are now at the point where we are tackling the first room. My old childhood room, soon to be the bedroom of my fiancée and me. On Saturday we exposed some square meters of the ceiling from the styrofoam and now the question arises – what do we do with the ceiling? Sand and fill and paint? Painter’s fleece 160g and paint? Plasterboard sheets on top? If plasterboard – what do I have to pay attention to with the existing panels? Do you possibly have any sensible ideas? I am grateful for any idea.
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Best regards,
Marcel

You have already mentioned some possibilities. All are possible. What should drywall achieve? It also needs to be filled and sanded. There are also stretch ceilings.

First, go down to the floor and check whether it is a warm or cold roof. Is the ceiling or the roof insulated, is there insulation between the rafters/trusses, are membranes or drywall panels installed? The Styrofoam was surely installed for insulation and not as decoration. Possibly, the insulation needs to be added or replaced, is the wood in good condition? The last thing is the design of the room ceiling. Maybe the room was not warm in winter and therefore insulated. So look, consider, get advice, and then carry out.

The wooden boards are attached at the top to wooden beams, and insulation is placed between the struts at the top. The roof is additionally insulated.

The room heats up quickly and cools down slowly when the thermostat is set to 0, I have seen dormer windows from earlier times completely without insulation. If everything is nice and warm, I would fill and sand the ceiling, wallpaper with painter’s fleece and paint it, and be proud of myself :)