Prebook trains or not

Öresundståg are regional trains and there is no need to prebook them. They can't sell out and the price is the same no matter when you buy the ticket. Prebooking will allow you to buy a seat reservation, which can be nice. But in my experience usually not needed as long as you avoid rush hour.

From Copenhagen to Gothenburg you have a choice as you've noticed. Öresundståg all the way, or Öresundståg to Malmö and change there to an SJ high speed train to Gothenburg. The high speed train option usually is a bit faster despite the change and often cheaper if you buy it in advance. Again, Öresundståg have fixed prices but SJ's price varies. The high speed train is also a bit nicer and has an onboard bistro.

And while first class can be nice, second class is perfectly fine.

Thanks.
When's rush hour?
Also do you think Denmark trains should be pre-booked, or is it the same situation (avoid rush hour)

Thanks

I'd suggest prebooking the trains in Denmark, that can save you a bit of money.

For Öresundståg, Skånetrafiken sell "group" tickets starting at 2 people with 20% discount, but they don't sell seats bookings. From Kalmar you can use their app in Copenhagen they also have ticket machines in the corner close to the exit for track 1-2. Kalmar Länstrafik also sells ticket but with less discount.

In Öresundståg around 50% of the seats are bookable. It is the red seats here: https://www.oresundstag.se/reseinfo/innan-resan/
Almost all seats between door 13 and 14 aren't bookable except for the animal/dog seats close to door 13. So since you start in Kalmar where the train starts just take a seat there. In first class choose seats on the left side when you enter. Öresundståg to Gothenburg often have door 21-24 too with no reservations at all, but check the displays some carriages may be decoupled en route.

Rush for long distance travel is Fridays and Sundays afternoons. The most commuters are between Copenhagen and Malmö in the afternoon, around 15-18. If the train is full they will stand in first class too. Between Malmö and Helsingborg there is commute in both directions mornings and afternoons. When going from Kalmar you pass Älmhult where IKEA workers might join you for their commute in the afternoon.

If you want an experience from half a century ago, take Öresund trains
or almost any other "local" train for that matter.

Experience from half a century ago? The oldest Öresund trains were delivered in 1999, so more like quarter of a century.