Having done pg wardrobe to a Christmas arrival, and working throughout the pg, what worked for me was:
I did get a mat suit (not Isabella Oliver but a specifically maternity brand that has since left Ireland) - a jacket that had a belt to loosen at the back as your front expanded, and a decent pair of trousers. PLain black.
I also got 1 mat shirt to go with that. But I had a few other shirts that were large enough to fit over bump for months, and then I just wore those open with a good vest under (see below) for the last bit.
I got 1 other pair of trousers, in navy, that were alright and that I could put a shirt or jumper over.
I used to wear a lot of my regular jackets, but left open (not an unusual look for me anyway) also.
As regards casual, I had 2 pairs of mat jeans that I adored.
I also got a pair of yoga trousers, for slouching, as the low belly on those went below the bump. I had a couple of belly warmers (not sure of exact name, but a strip of elastic'y fabric that went around the bump, partly supported but also covered any gap between waistband and bottom of tops) - from H&M. They were very useful.
I had a bunch of mat tops in plain black, plain white and a couple of colours.
A few of those were long sleeved, a lot were short or just "across shoulders to edge of the arm" sleeveless (so not strappy). Most of the sleeveless ones were "maternity vests" from Mothercare - but meant I could wear a shirt open over it, and be completely covered up and respectable without melting. Or a cardigan in casual time. I did get one that was more strappy vesty type , which wasn't great and had to be under a full cover-up/top/jumper.
I sacrificed 2 of my beloved black M&S poloneck jumpers to pg as they were stretchy, and they went out fine as needed but only had to dump one after (the older!) as the other went back into shape fine.
I got a long black cardigan, that tied with a little string (I'd just get one of the regular ones that are open now, or maybe a pg one if feeling flush) and that meant I could mostly get away with not buying a coat.
I ended up, even when there were frosty mornings, just layering up. So a pair of mat tights (coming over belly) under my trousers, black sleeveless top, belly band, wool polo neck over that, black cardigan and my old black wrap thrown over the lot, with a pair of gloves and a hat, meant I was still toasty outdoors but could strip down as needed indoors.
I tended to stick to black a lot, but that was a staple anyway at that time. I added a few more chunky necklaces and scarves to my collection to give it a bit more colour and variety - which are still worn now.