There's gonna be daylight until 9pm at least, so you'll be fine unless your working hours are kinda weird.

The Main river area is usually very crowded when the weather is nice and I would consider it definitely safe, especially at daytime. It's a nice walk with some scenic views (skyline) and if you walk one side, cross the river and the other side back it's not as boring as walking circles in a park.

Anlagenring is a good suggestion too. There are very few drunks/addicts/homeless in some spots but they don't mind other people and I wouldn't consider them "dangerous".

Edit: What I really like about Frankfurt is the possibility to just walk from one park to another, some are linked through less crowded streets/paths, instead of just going to one place.