March in Alsace is rideable, but it's the shoulder of the shoulder. Expect 8-12C in the afternoon, cold mornings, and a real chance of a wet day or two. On an e-bike the cold matters more than the rain: budget for roughly a third less range than the quoted battery figure, and carry the charger with you rather than leaving it at the hotel.
For a hub, Colmar works and is the obvious choice. You're on the vineyard cycle route within ten minutes, and Eguisheim, Turckheim, Kaysersberg and Riquewihr all sit inside a 25 km loop. Trains north to Selestat and south to Mulhouse take bikes, so you can shortcut a leg if the weather turns. Selestat is the quieter alternative and puts Haut-Koenigsbourg and the Ried plain within range.
The wine caveat for March is the real one: a lot of the small family domaines in the villages are open by appointment or on reduced winter hours, and some don't properly reopen until Easter. Email two or three ahead rather than turning up. The ones in Riquewihr and Ribeauville that keep tasting rooms open year-round are the exception, not the rule. Whites are the region's strength anyway, so you're well matched.
With 18 days and Vimy already in the plan, four days in Alsace from a single base is the right call. Moving hotels in March gains you very little.
