What if Clay is more successful, and this more than OTL-level of internal improvements start in the 1830s? Some of this program is stuff the South would have wanted, and one can see how more infrastructure could have lead to more industrialization in the South.
(...)
The thing is, that removes the secession over slavery (maybe, possibly) but then what else is there to secede over? Its almost as if Slavery is the one big question here, and all else is Window-dressing and after the fact myth-making...
Wait... you basically come up with an ATL that serves to remove the legitimate OTL grievances of the south in regards to northern interests (high tariff, internal improvements for the north, subsidies for northern business) prevailing over southern interests (free trade, any internal improvements to be more evenly spread across the nation, no subsidies for specific businesses)...
...and then you jump from
that to "Its almost as if Slavery is the one big question here, and all else is Window-dressing and after the fact myth-making"?
Bit of a weird conclusion, because it would only actually hold up in your ATL. The fact is, "everything else" covers a whole lot of issues that are far more than "window-dressing" and that are certainly not "after the fact myth-making."
From the very outset, the USA were faced with competing visions of what America should be. Look at the struggle over remaining confederal versus adopting a federal constitution. Look at Jeffersonian ideals versus Hamiltonian ideas. From the outset, you had a major strain in American politics that believed in small government, free trade and decentralism. A mostly southern strain, ultimately. On the other hand, you had a strain that advocated a powerful government, economic protectionism and centralism. And that was mostly a northern strain.
Those differences could be addressed, and more easily than slavery could be (unless slavery is tackled by the 1830s, when it was far less of a divisive issue). But even without slavery in the way, the other issues are by no means "after the fact myth-making;" and they could very possibly lead to the union breaking up, if not addressed adequately.
Something very hard to accomplish in most cases, I fear. Although, with an early enough POD, you could have Georgia be a free province (as was
requested in OTL) and a potentially abolitionist Virginia (maybe not going past the "indentured slavery" phase) in the late 17th-early 18th Century, likely hemming in the expansion of the institution hencewith. Just an idea...at any rate, it would help with the OP since it'd take that disgusting elephant out of the room (unfortunately, most Civil War scenarios with a sectarian slant seem to fall susceptible to this dichotomy, albeit not without reason).
Hey, cool! I didn't know that about Georgia!
There are several ways to get rid of slavery, of course. Or at least to crucially weaken it, making it easier to get rid of later. That said: tackling it "too early" would, for the purposes of this discussion, create so many butterflies that we would no longer be able to say anything sensible about American politics in the 19th century.
One fact people often ignore is, of course, that cotton had its booms and busts. As of the 1840s, it was IOTL enjoying a very long "boom" that lasted up to the civil war. This entrenched slavery. But in the 1830s, cotton prices were far lower. As far as I can tell, given the right circumstances, the 1830s would have been a great time to ditch slavery once and for all. (Not that it would be easy; but far easier than it could conceivably be at a later date.)
Also, not to be that guy, but have you checked
THIS TL out? It's the same author of the English Brazil TL

.
It's a very well-written TL, and I've certainly enjoyed reading it - but I do consider it too optimistic. It posits an earlier (and, crucially, shorter) war; both these factors would help in getting the south to be less... rabid about slavery. The later in time, and the higher the cost of war, the more embittered and entrenched they became about the issue.
But a war that's a little earlier and a little shorter can only help so much. So I'm not so optimistic about the southern willingness to get rid of slavery "eventually". Oh, Virginia and several other states, sure. With OTL's civil war that would be difficult, since the OTL confederate constitution made getting rid of slavery nearly impossible. But with a slightly earlier civil war, and a slightly different confederate constitution, I could see Virginia, Tennessee, Arkensas and Texas getting rid of slavery before 1900. The Deep South, though? I have my doubts. You'd need an earlier POD.