A frozen goose is perfectly lovely and not too expensive (£20ish for a moderately sized bird, IIRC). We buy ours from Aldi. We’ve paid loads more for fancy fresh birds in the past and we’re perfectly happy with the quality of a frozen one.
A couple of ducks would make an amazing Christmas dinner. If you buy them when you see a good deal and freeze them (if you’ve got the room) they don’t have to be expensive.
For turkey or goose we stick to bog standard dried sage and onion stuffing as we’ve decided we like that the best (again, have spent lots of money, time and effort on fancy ingredients for fancy stuffings in the past), we just upgrade and buy a ‘best of’ for Christmas.
M&S premade red cabbage is our quick and easy side, we just microwave it on the day. We make 2 other sides ourselves from scratch.
A really good point is that it’s the sides that really make a Christmas dinner. Spend a little bit more money, time and effort on those and your meal will instantly become more special. As you said, if you decide a chicken is your best bet for the roast, the sides will make it feel just as Christmassy. However goose is delicious.
For treats, buy less but better quality. It will cost you the same, or maybe slightly more, but is well worth it.
Instead of 5 boxes of nasty Quality Street/Roses/whatever, buy 1 or 2 big boxes of nicer chocolates. Same for biscuits.