So, is the Mini 3 Pro not eligible for the sub 250g classification in the EU just because DJI sells a Plus battery, even though you never purchase or use one?Like with the Mini 3 Pro, but there's probably the catch you rightly point out: the Mini 3 Pro is with the standard battery < 249 g but DJI features a battery plus, which then adds flight time but surpass the 249 g threshold.
The Mavic 3 Pro is in standard configuration C2 with >= 900 g but < 4 kg. So it's most likely that you may exceed your MTOM with additional batteries but your original level is then void as with the Mini 3 Pro, but I think, if you specify your drone with C2 and then reduce weight, it's not possible to fall below the C2 rating.
Not an expert here, but I guess, that could be the reason for DJI not doing any battery tricks here.
Here are EASA rules:
The classification should depend upon your configuration and your use, and not some hypothetical possibility. China didn't prohibit the Mini 2 when used with the smaller battery to keep it below 200g, even through a bigger battery was readily available from DJI that brought it to 250g. Why should the EU be any different?