There are so many accessories to consider. I am going to list some of the items you may want to consider, and some will have links to products that I have found to work well:
helmets
fenders (these will keep you clean and dry, but will make it more difficult to secure on a bike carrier)
lights (while you can add lights after purchase, getting them built in offers integrated control and normally run off of the bike's battery)
rear rack (to carry items)
Car Carrier: Thule EasyFold XT - holds two eBikes, folds compactly for storage, works with fenders, simple to use knobs that lock with a key
bags: (panniers-see below, trunks, saddle)
Panniers: Ortlieb Back-Roller Urban - easily clip on and off, waterproof, well made
Ortlieb Commuter Inserts for Panniers - transports laptop and water bottles
Ortlieb Packing Cubes for Panniers - keeps the stuff in your panniers organized
locks and chains (eBikes are often stolen as they are high-value targets)
Phone Mount: Quad Lock - easy on and off and a secure mount (but you have to use their case, which adds bump)
Mirror: Hafny E13 - large display
Tools: Crank Brothers M10 - compact multi tool with all of the drivers you need on the road
Floor Pump: Serfas FMP-500 - simple, sturdy, pumps on both strokes, dual fit head fits Presta or Schrader valves
Computer Upgrade: Bosch SmartphoneHub - While eBikes will come with a computer, I opted to upgrade the simple Bosch Purion computer to a more advanced computer that integrates with online portals such as Komoot for route planning and Strava for route tracking. It also allows you to control your phone for music/podcast playback and calls and texts. It will give you turn-by-turn navigation and even indicates if you have enough range on your battery to make it home.
Dual Battery: Bosch Dual Battery Kit - I added a second battery for twice the range! It is integrated into the bike so it works as a system.