We struggled ourselves to fit a maxicosi carry seat into out 3 door ed30, like you it wasn't so much getting the base and seat to fit in the rear but more getting the seat in and out of the car past the front seats, was hard enough with the empty carry seat so no chance with a baby in too!
Had a bit more luck with a graco carryseat a friend gave us but ultimately it was so much of a faff that we chopped the car in for a 5 door..
Imo you don't have to get rid of the 30 in a rush, maybe try some other seat solutions at Mothercare first etc
Also I'd have to agree that the bolsters in the front do make getting some car seats to fit correctly! The back can be a pain too tbh, after our little lad had outgrown the carryseat type we had trouble finding a regular car seat that I was happy was sat 'in' the rear seat rather than 'on' it, spoke to VW who recommended the Recaro young sport and it honestly fits perfectly