Ok, so it sounds like you're thinking this thru pretty well...
I'm not saying don't buy a .40S&W ..../ everybody should buy and shoot whatever they want ! ....and I have identical platforms in both 9mm and .40S&W...( Sig 239's, Sig 226's and 1911's )......and over the last several years when a good deal showed up ...I just picked them up as the budget allowed.
As it turns out for me ....I shoot a lot of 9mm in a 5" 1911 ...( I carry an identical gun in .45 acp). I usually run about 6 boxes/week thru the 9mm version and two boxes/week thru the .45 acp version. So that is my "Tactical Practice" for the week....double taps, reload drills, in and out of holsters...multiple targets...etc. Since I reload - 9mm is relativley cheap to shoot ( $6 a box ) ....gives me some recoil but doesn't beat me up (I have some arthritis, hand, elbow, shoulder issues )...and a few boxes of .45 acp ---just because I love the caliber ( and I'm an old school 1911 guy - Wilson Combat 1911's - and I will always carry a 5" 1911 ).
For the rest of my shooting ...once in a while its one of the Sig's in 9mm or
.40S&W - or I make it a Sig day ( where I exercise the 239, 226, X-Five, etc).....or I make it a revolver day - where I exercise a variety of S&W revolvers model 19's, 27's, 66's, 686's - 29's, 629's ....in .357 mag and .44 mag...../ and somedays its a rimfire day ...../ and its a mix of "bulls eye" shooting and Tactical Drills....but its more about having fun / than "practice".
But honestly, the .40S&W caliber for me,.gets lost a little in the mix ....my usage by caliber is about
20% .45 acp
15% .357 mag
.....( and less than 5% .40S&W )....
and its not the guns - because I have some great guns in .40S&W...( Wilson Combat 1911, Sig X-Five L-1 model, all stainless Sig 226, a little Sig 239 ...and its not the recoil, although the .40S&W is a little snappy - and I load a 180gr bullet exclusively for that caliber...its mostly because I like shooting the 9mm, .45 acp and .357 Mag...and the .40S&W just gets left out.
Now, if this ammo shortage is not short term....that's another reason to add a .40S&W if you want to ....but I think its short term. I think as soon as the middle man/profiteers/hoarders ....run out of money, ammo will be readily available again in all calibers...maybe at old prices, maybe not....but I think it'll be available.
But don't let me talk you out of a new gun .../ this is a gun forum ...buy as many as you want !! ( I have !! ) ....and I have virtually everything I want - with about 25 semi-autos / and about 25 S&W Revolvers - and one Single Action Freedom Arms...