The batteries themselves are surprisingly simple once the new ground rules have been fully understood (mostly that you won't outrun the electrons so respect the bright orange stuff and test, test and test again before licking any of it

The EV technology brings a whole new way of diagnosing and repairing vehicles that both main dealers and the independent network of garages have adapted to. Remember.... technicians will often join/leave both dealers and independent garages so much knowledge becomes shared outwards from manufacturers informally in this way.
EV tooling is widely available and organisations such as The IMI provide training to local garage staff that, although perhaps not as detailed on a given brand as what I deliver, does make the technicians safer and add to this the standardisation of EV systems thanks to SAE etc, the local garages should still be able to stay afloat.
They'll need to invest in tooling, staff training, diagnostic computers etc but.... I'm not so sure that pure EV will be a longer term solution (personal opinion only), fuel cell is likely to be the next big step which has EV systems within so we really are at the start of a transformation period that will see the investments made now bear fruit for years to come.
Much like when carbs/points began to be replaced by EFI, ignition modules, ECUs and we all thought we'd be losing the local garage.... we didn't thankfully