I think everyone should have a Noodler's flex pen to dick around with. The QC on the nibs is a little... interesting, but there's so much you can do with them. You can play around with the mechanics by adjusting the nib and feed position, what difference the breather tube makes, eyedropper conversions that (in the case of the Ahab) can hold up to 6ml, and a bunch of other stuff. You can get out a dremel and try ease my flex or a tomahawk mod (You can get spare tipped flex nibs for $5 and untipped/non-flex nibs for something stupidly cheap). You can even try modding the feed. They're great fun. That being said, don't expect the kind of frustration-free performance you get out of your VP. Also, some people say they smell bad, but I like the smell and you can take away most of it with a good cleaning.

Pelikans have a reputation for being butter-smooth but I've never tried one personally. Same for Watermans. I don't really know very much about maximizing smoothness, to be honest.

If you want a true flex for under a zillion dollars that isn't frustrating or require hours of modding, you might look into some vintage pens. You have to be really careful when buying vintage flex because a lot of people have very different definitions of different kinds of flexible - a wet noodle to one person might be a semiflex to another. Good places to start looking, although none of these will always have flex nibs (and you'll definitely find different ones with varying flex levels), is with Eversharp Skylines, Wahl ringtops, and Mabie-Todd Swans. If you're looking for semiflex there are a whole bunch of other options out there you can easily get for sub-$100 fully restored.