I think this is a fair suggestion. Today there is nothing really appealing to me in the decentralized tree. Even if the last reform has a really kickass name, it feels like a toned down version of the Revoke the Privilegia one.
On my part, I would only chose the decentralized tree for a RP reason. This is fine by me, I love the fact that eu4 is very a asymetric and somewhat unbalanced game. But I feel that, as OP said, when you go through the hassle of managing the HRE for two centuries, the final reward should be glorious and this branch of the tree feels a bit lackluster as you just get a glorified CB.
To complement OPs suggestion, my suggestion would be that the current decentralized final reform should be the second to last in the decentralized tree and the vassal swarm would move to the final reform of the decentralized tree, but in a slightly different way.
Indeed, to my mind, the vassal swarm feels a bit too centralized in term of flavor so I would suggest to change it. An example I really like would be for the final reform to force an alliance between all princes costing no diplo slots and giving a massive bonus for joining offensive/defensive war between the various princes. This would give fun endgames as you may be able to see the HRE utterly destroying the known world even without your direct intervention as HRE members (even tiny OPMs) would be able to call all the HRE in their offensive war as well. Finally, I think this would really respect the decentralized flavour this tree should have as this button would empower the HRE as a whole, not the emperor only, which makes being the emperor virtually irrelevant which I think should be the point of a decentralized HRE.
This would also solve another problem in the Centralized tree. Today, I don't feel anything was done to make the final reform, forming the HRE as a unified country, feels stronger than the vassal swarm. Yes, you correct the atrocious border gore created by unrelented vassal swarm conquest, but this is a button I usually press at the end of my game for the final screenshot, not for gameplay reasons, as its mostly a downgrade in power for your country. Removing the vassal swarm from the centralized tree would make unifying the HRE a really enjoyable button to press as it would be a real step up in power from the previous reform.
