Sagging Waste Line - Eastern PA

Hello! Looking for some possible solutions (if any) for a sagging waste line.

Under the concrete floor in our first floor is a sagging waste line that I was told holds about 1" of water regularly due to the sag/ settling. It hasn't given us too much of an issue but yesterday it backed up into the first floor shower. The drain cleaner said there was grease trapped in there, he blasted it out and it is flowing properly,

The main plan is to have our plumber rip up the 5 foot section of the first floor and replace the pipe, but I'd love to hold out until the end of summer. Is there anything I can pour into the first floor bathroom shower basin to ensure the grease build up, if any, is cleared or at least being managed as best we can.

The shower it is backing up into and where the issue is is about 20 feet from one another so whatever I do I will need to make sure if enough to be effective going down the drain and out 20-30 feet.