Many interesting analog front end options to choose beginning from your short list and by way of others mentioned here.
How important or beneficial is a well-implemented vacuum hold-down system?
Personally, vacuum platter hold down systems have always proved more fuss and trouble in my setup than they are worth, but to each his own.
My $20k short list -
(or perhaps a restored Sp10MkIII) with Artisan Fidelity Panzerholz plinth (Both are leaps and bounds ahead of a stock MkII) can be made for installing a pair of arms.
Kuzma Stabi R - Single to Quad tonearm setup with Kuzma 4Point arm. Value-to-performance high and agreed per gleeds
comments about AF.
From your own list -
Tech Das Air Force V - If you must have a vacuum hold down feature and prefer a more relaxed presentation. (Although I see your budget has changed since the initial thread post)
"Are the Garrard 401 and SP10mk2 set-ups I own now "good enough" already? I know there are some out there who feel these vintage tables, well restored and set up, can be a platform for almost any arm or cart. Still, I'm curious if I can improve on what I already have."
There is still plenty of room left for improvement with the Garrard 401 setup you have which will take you up to your budget limit, but it is worth it if you enjoy the idler drive character and are willing to explore an end of the road
solution. Your Sp10MkII would need a great deal of modifications and upgrades to hear any difference at which point you are really better off investing in a new Sp10R or MkIII instead either of which are fundamental upgrades.
Are there other factors or pros and cons to the above I'm missing?
Many factors are at play (no pun intended)
In truth, all of the turntable recommendations mentioned are unique, each in their own way in terms of inherent design and musical character. You simply need to narrow down your priorities first and then make a move.