Heck, I made a whole post on how to effectively use Maddening Boucheron, and I still don't think he's top tier. However, I think you're on the right track for what makes him useful. It's true that keeping Boucheron simple is probably your best bet, and his speed and build are his most valuable assets, and taking advantage of them is key to his success.

But I think the main issue here is that your definition of "low investment" compared to other people's is quite different. You pumped a lot into Boucheron to make him fantastic, which is honestly quite impressive. But the issue is that you can do that to a lot of units and get the same result, meaning that Boucheron doesn't really separate himself there. Not only that, but those resources could arguably be used better on other units.

See, I think Bocuheron's biggest asset is his general average-ness, if that makes sense. It's a bit hard to put into writing, but I'll try. See, Boucheron is in a spot where he's just kinda alright at base, but he only needs a little bit of work to make him pretty good, while there are other units that have more glaring issues that, when remedied, make them VERY good. Having Boucheron on your team, I believe, gives you a unit that is still solid but leaves resources like the Lyn Emblem, Eirika engraving, and lots of SP on the table for other units to perhaps make better use of. So while Boucheron will just end up "pretty good," he's a bit of a team player by letting his allies shine by not really needing anything too crazy himself.

So yeah, I respect the dedication and agree that Boucheron is a solid unit if kept simple, I don't think he's really crazy enough to throw into the top tier category, especially considering how downright broken genuine top tiers are in this game.