My sister is a pharmacist, so I asked her for advice. When I mentioned your situation, she said that 25-plus years is a really long time, and the brain and body get very used to it. So, lower energy and mood are unfortunately super common and expected even with a slow taper.
She also said that brain zaps are a classic sign of the body adjusting, and since Bupropion has a very long half-time (especially the XL form), it can take a while for the 300mg dose to fully clear out and stabilise before dropping further.
It sounds like you're doing exactly the right thing by working with your provider and taking it slow.