Yeah, New Glarus, as far as I know, is still solely sold in Wisconsin and do not have any public plans to expand their distribution. I have seen it in Illinois at a liquor store, but it was, how shall I put this, not an official product, and only was there for a week. (But it was in the regular beer cooler.)
Steigl's Grapefruit Radler tastes like grapefruit soda. I don't really detect any beer flavor in it, and it's about 2.5% alc. But it's a nice grapefruit flavor, not too artificial tasting and not
too sweet. It is very nice for a summer's day.
Based on you liking some wheat beers and Guinness and your general distaste for bitterness, I assume it's mostly the hops that you don't like. (Also, while Guinnesss has a reputation for being a "meal," it's a relatively low-calorie beer at 125 calories per 12 fl. oz. That's less alcohol than an Old Style, and the same as a Michelob Light, believe it or not.)
I would definitely second the recommendation of going to Goose Island or similar brewpub and ordering a flight of beers to get an idea of what different styles taste like. Also, remember that at most places, you can ask for a taste of beer if you're unfamiliar with it. I've never asked for more than two or three tastes before deciding on a beer, but in a beer-centric bar, the staff there are more than happy to match you with a beer you might enjoy.
I suspect that you should be able to find something in the Belgian beers that you like. Belgian beers tend not to be hoppy, emphasizing more malty and yeasty flavors (which tend to be banana-y or clove-y). They are more "complex" in a certain way, as they don't have a tradition of beer purity laws, so you can find all sorts of spices, fruit, candy sugar, etc., in their beers, depending on which style you go for.
I would try the following, easy to source beers.
Goose Island 312. This is a light, American wheat beer. Low on hops, low on the yeasty flavor of German and Belgian wheat beers, refreshing. I didn't like this one the first few times I had it, wanting it to be much more yeasty, but I've grown to enjoy the style. My favorite example of American Wheats is Bell's Oberon. Also worth trying if you want something with a little more flavor than the 312.
Hacker-Pschorr Hefe Weisse. A German wheat beer. Wheat beers can be love-it-or-hate-it beers, but it seems you enjoy the wheats, so I'll keep going in this direction. It's going to be a lot fuller in flavor than the 312, a little bit cloudy, and has a distinctive yeasty taste that brings bananas to mind.
Hoegaarden. Continuing with the wheats going the Belgian direction. This is a Belgian wheat beer brewed with coriander seeds and curacao orange peels. Light in color, cloudy, yeasty (clove-like). This is the beer style (witbier) that Blue Moon is based on. I'm not a big fan of Blue Moon, but I like Hoegaarden. Might be worth a taste
Samuel Smith Nut Brown Ale. This recommendation is a little bit of guess, but since you like Guinness, and it's a bit heavy for you, a brown ale might be more to your liking. Low in hops, with a roasted nuttiness in the malt. I'm personally not big on the brown ale style, but this is a good beer and I've discovered it to work well with people who don't typically like beer.
New Belgium Fat Tire. I actually dislike this beer, but I think you might like it based on your description. Once again, a crowd-pleaser, light in hops, medium/medium-light body. It's got a pleasant, caramel/toffee sweetness to it. I personally find the flavor a bit too cereal-y for my tastes, but it is a solid beer.
Now I'm going to go in a completely different direction:
St. Bernardus Abt 12 (or Rochefort Trappistes 10). These are both very heavy, highly alcoholic beers (at about 10.5-11.5%). This is a recommendation if you want a beer that is more like a complex, heavy red wine than something to quaff on a hot summer day. Malty, sweet, fruity, yeasty, with flavors that remind me of currants, berries, raisins. Mouthfeel is a bit thick, carbonation is quite low. I'd even describe these beers as port-like. Very different than what you think of as beer, which is why I'm taking a shot in the dark with this recommendation.
The basic fruit beer recs have been made, but I'll point out, if you are in Wisconsin, do pick up New Glarus's Raspberry Tart or their Belgian Red (the sour cherry one.) These are AMAZING fruit beers. Just incredible. I didn't think I liked fruit beers until I had both of these. They just have an incredible freshness of fruit flavor to them. Lindeman's lambics taste more like pop than beer to me (plus they are artificially sweetened with Ace K--at least that's what listed on the European labels.) Oh, and New Glarus's apple offering is equally wonderful.