If the oak is finished to 25 mm you have enough depth to use dowels. The great James Krenov used dowels for his display cabinets. A simple dowel jig or even dowel pops will help. The only downside is that the endgrain of the top and bottom will be on show. I wouldn't practice dovetails on what is to be a finished piece - you are guaranteed heartache. As with all cabinet pieces a set of sash clamps is obligatory.
Dowels were my original solution, I have a dowel jig as well. I will practice dovetails on test pieces and If I will not get it right in few times, I will use double rabbet with dowels.
Sash clamps are a bit of issue for me right now. I have those quick grip clamps up to 750 mm, but I will not be able to clamp the length of the cabinet that is 1200 mm wide. As one good sash clamp is 50+€, and now I am not able to give another 200-300 € for clamps, so I need to look for some dyi clamp solution online.