Downpipes to stormwater is to small - gutters overflow

I have 8x 90mm downpipes all connected to a single 90mm stormwater pipe that’s is about 40m in length to the gutter

When we have decent rain this can’t keep up and my gutters overflow

The obvious thing to do is replace the single 90mm pipe with 15m but it’s under concrete

If I ran another 2x 90 stormwater pipes (like paralleling the stormwater pipe) would this have the same effect ? Or is there a better option?

I can’t cut the concrete and replace that pipe so looking for other alternatives.

I was thinking ever 2 downpipes that come down I can put a tee piece in and run another 90mm storm water so there would be 3 running off, not just the single installed like now.