Starlink will probably do better than 5G for access to rural areas because you don't need to build out a physical cable. In highly dense urban areas with large numbers of simultaneous users, 5G is a better fit.

Have the 5G towers connect to Starlink rather than running the cabling to them.

That doesn't really fix the total bandwidth capacity issue though.

I wonder if a tower with a larger array of Starlink antennas could connect to multiple satellites and get more performance than the pizzabox-sized consumer receivers

In many cases you can link base stations to other downlink points using lasers or really tight beams though. It's not like they can only go upwards.