... BTW "GB" no longer accepted must be "UK" - (Brexit?). Maybe an exception for number plates with Country Code.
It was entirely the UK's decision to change what had been in place since 1903 (I believe). Nothing to do with the EU nor Brexit
per se.
The UK notified the UN that it intended to change, most accounts reckon it was something to do with wanting to make Nor'n Ireland feel included (not included in GB of course).
A number plate showing no country of registration is of course LEGAL to display, as is a plate showing GB or the EU ring of stars or a St.George flag and GB etc, however these do NOT satisfy the legal requirements when abroad and in such cases a separate UK plate must ALSO be shown.
A number plate showing a Union Flag ("Jack") and a UK symbol satisfies the requirements without the need for a separate UK plate.
Countries' police forces vary in how they enforce or ignore such things, Austria are pretty strict for example and you will very probably be fined for failing to comply.
As for bulbs, I generally carry a box of a selection of bulbs which may or may not be appropriate for that car, no-one is ever going to check whether your car uses a 380 or a 580 stop/tail bulb (lamp). Hopefully they are right. The only ones I make sure are correct are headlights. I have had those go both in cars and on motorbikes, definitely a problem at night when you only have one headlight and it doesn't work!.
The other thing is to make sure you carry hi-vis jackets/waistcoats for all passengers and they must be IN THE CABIN, not in the boot. People often have them put over the seat back so they are visible, avoids any questions. You MUST wear them if you have to stop on the highway and get out of the car for breakdowns etc. so they have to be reachable inside the car.