Hello,
what surprises me at first is the low amount of equity available. So far, you probably haven't paid much attention to money. No accusation, I had that once too :-) Now you want to set aside 600-800 per month? Sure, go ahead - not a bad plan per se. But look at it realistically, also plan for money for a new car in 5-10 years etc.
Regarding investments: I currently find building savings contracts great in this interest phase to secure the money.
Funds: The DAX is too high for me right now, I would be cautious with it. Either take another building savings contract or a safe investment. I am currently doing well with containers, but the money is tied up for 5 years first :-) Safe investments are just lousy right now - but does that mean taking a risk now? Better secure a good interest rate through a building savings contract. Pay attention to the repayment during the loan phase - don’t choose it too high!
Riester: I am doing it right now too, I think it’s good. Because paying off the construction loan AND Riester simultaneously isn’t exactly easier.
Roger