2: Prices increase, and you'll have a lot of work with customs.
Why would you have "a lot" of work with customs, and increased prices?
We buy almost everything from Digikey, USA, because it seems to offer the best mix of being cheap enough, good service, and good inventory. FYI, USA is not part of EU. There is absolutely zero customs hassle. Free shipping. Everything's handled automatically by the shipping company. And, they have a bank account in Europe, and they can sell with no VAT to the EU customers, like any EU-based company.
Why would post-Brexit RS be different from Digikey?
Why Digikey has't relocated to EU, if being in EU is so important?
So maybe there is this option:
3) They'll continue doing business like every other non-EU distributor does now,
possibly by opening a minimized EU branch office somewhere. Edit: As others have informed, this has already been taken care of, as usual with big firms.