Silly turntable question

I suspect that a change to a 12" arm will be subtle, and not necessarily for the better. Depends on the cartridge you use.

I agree with those who suggest that you simply get started. However, to answer one of your questions: there is quite a difference between the 9" and 12" SME arms. I spent two weeks directly comparing the SME V to the SME V-12 on an SME 30/12 table using the same cartridge and cable.

Moving up to the 12" arm was like going from a highly resolving two-way compact speaker to a three-driver, three-way floor standing speaker. Extension and weight improved as expected. But unlike with some such speaker changes, there was absolutely no less coherence or sense of balance. There was just more information presented in a cleaner and clearer way and there seemed to be less distortion. IMO, well worth the cost to upgrade.

But changing the plinth and selling your unused SME V just to accommodate a new 12" arm may be more hassle and expense than it's worth right now. Check your priorities, get started, and join the fun.

IMHO, the Panasonic obsidian base does not sound that great. My recommendation, first if you have a regular SP-10, sell both it and the base and get an SP-10 MK2. There is no comparison here. Make, or buy a nice Baltic birch ply plinth, or if it's in the budget, go slate. All of the above is going to be better bang for the buck than a 12" arm. But if you have the dosh left, go 12". As my mistress says, size does matter.

Dreaming about it is always better than doing.

So I say yeah, sell the arm and buy the longer one. THEN sell the plinth and spend a few years finding someone new to make you a custom one.
If you stall long enough, maybe your table will never get finished.
But if it DOES seem like it may near completion, you can always waffle, and buy a second short arm again, then again sell the plinth, and get another new one made to handle BOTH arms..
All this should take at least another seven years..
And once you have a table, two arms and a plinth all assembled, you can spend forever deciding what cartridges you want in your arms.. and what phono preamps to use..
This could take a lifetime if you work hard at waffling.

But Elizabeth, isn't this part of what we do as hobbyists- consider changes and enjoy the process, from conception to implementation? I didn't read the OP as someone who deserved thrashing for failure to implement after years of kicking around the idea....