If your ETF is "ausländisch thesaurierend" ("foreign accumulating"), declaring it will be a bit more complicated but not that hard. Would you mind explaining your situation a bit better? Maybe I'm missing something (and I wouldn't want to, since I'm looking into this topic myself at the moment).
Do Germans not have any practical way to invest in accumulating funds?
Just beware of taxes.
https://www.charles-stanley-direct.co.uk/ is a good shout, but there may be better depending on how you'd like to invest.
For non-EU/US residents I got a recommendation for http://int.tddirectinvesting.com/
Is there a value add that I'm missing?
http://www.hl.co.uk/funds/index-tracker-funds/wealth-150
For example, that provides the FTSE all-share through an L&G fund:
http://www.hl.co.uk/funds/fund-discounts,-prices--and--facts...
Which has fractionally lower fees than the Vanguard equivalent:
http://www.hl.co.uk/funds/fund-discounts,-prices--and--facts...