Some digging got me https://www.verkaufsoffener-sonntag.com/ladenoeffnungszeiten..., which tells a history of shopping time in Germany (although I don't know how accurate). There are minor differences on the wiki page about the same topic (https://de.wikipedia.org/wiki/Laden%C3%B6ffnungszeit#Geschic...)
According to that page the first laws of that kind in Germany were established in 1900 and were amended by exemptions for grocery stores - and jewish shops (to open on sunday instead of saturday).
Merchants (presumably including the jewish ones? that article doesn't tell) worked out a new scheme in 1911-1919 that introduced common closing times and the "calm sunday" to coordinate opening hours. While there were other changes in the meantime (mostly around closing hours), the Sunday provision stuck since then.
(Also note that there are separate laws for working hours and shop opening hours. The citations up-thread were about working hours, while this post refers to shop opening hours)
There was a constitutional court decision ~10 years ago as the state of Berlin (shop opening hours are a state matter) decided to relax the rules. "10 Sundays across the year, and all Sundays in December" (in the lead-time to Christmas) were supposed to be shopping days. The churches sued, and while religious aspects came up as well, the main thrust was about the social state principle (ie. "One full month with no mandated rest leads to exploitation"), and so the rules where tightened somewhat.
I guess a state could enact per-shop rules (ie that a shop could decide to open up to 6 days a week as to their choosing, and to avoid exploiting the regulation, the schedule must be held up for at least a year).
As Berlin seems to be the hub of Jewish life in Germany, that looks like a good place to lobby for that, but that would also require some changes to §9 / §10 ArbzG as linked upthread as an open shop on sunday without people working there is rather useless.
https://www.gesetze-im-internet.de/arbzg/__9.html says "no work on Sundays or public holidays from 0:00 to 24:00", plus a few exceptions where the 24 hours window can be shifted by up to 6 hours (but has to remain 24 hours long).
This implies a legal work week from Monday to Saturday within which employment contracts can schedule their working hours (following more rules, eg. not more than 10 hours at a time, mandatory breaks of a certain length within those 10 hours, and at least 11 hours of rest between two days of work).
7-8 hours a day Monday to Friday is currently normal, but deviations are possible within the legal limit.
https://www.gesetze-im-internet.de/arbzg/__10.html enumerates the exceptions to the "no work on Sunday/holidays" rule. If your business doesn't fit into these exceptions (as is common for regular stores), you're not allowed to open on Sundays.