Years ago I worked at a garden center that sold a BRIX fertilizer. Expensive, but I got a tub or two cheaper. It was good stuff, worked nice in the containers and garden beds. Had healthy plants and good harvests. But I can't say if it made the fruits sweeter. It was during the years I ran color trials on tomatoes and scoville tests on peppers, so I was trying a lot of new varieties. I was growing to compare against each other rather than testing the difference between the same kind of plant grown with different fertilizer conditions.

I've heard of adding sugar to soil to sweeten- it's a carbon. It does feed some micros that can in turn be beneficial to the health of the plant. A healthy plant produces tastier fruit. But it does not directly impact the fruit itself with sweetness.

I've heard of biochar for sweetness too. But it's not directly impacting fruit with a sugar boost either. It can improve the soil in various ways that can lead to a healthier plant better able to produce good fruit.