I'm of the "use what's issued" group.
Sure, when I was a younger cop I was VERY concerned about such things. I was a firearms enthusiast and fancied myself knowledgeable and experienced.
After a career in LE, with a lot of time as a firearms instructor & armorer, it wouldn't concern me anymore. I adapted (with medium size hands) to all the different revolvers and pistols that were issued over the years. As an armorer for a few different makes/models, they all have their advantages & disadvantages.
Now, if you have VERY small hands and can't sufficiently grasp the G22 and function the trigger to make consistently accurate hits? Do they offer an option for the occasional "disparate impact" situation (meaning a small-handed shooter who may also be recoil sensitive to the .40 S&W)? If it's "buy your own", you're back to square one.
If they issue the Glock, and have one or more Glock armorers on-staff, then the periodic inspections, armorer level maintenance and any repairs are on their dime. If they don't have an armorer on-staff for whatever optional pistol you may choose, then it's left up to you. That can create some difficulties for periodic inspections (beyond field-stripping for cleaning & routine user-level inspections). If a repair is needed, and it requires the gun being returned to the manufacturer for repair ... and then you have to carry the issued G22 (and holster) in the interim.
Also, if your personally-owned pistol is actually used in a shooting, you're not only going to probably see it kept in evidence for the criminal and civil proceedings, but now if any questions or questionable issues may exist regarding the weapon, then its condition and servicing may be left up to you to explain (instead of the agency armorer).
If a personally-owned weapon is lost, stolen or damaged during the course of your daily activities (on & off-duty), then the cost of its replacement is on you ... and you get to carry the issued G22 (and holster) in the interim.
If it's their G22 that suffers a problem? Hey, they'll give you another one that looks just like it.
Another option that may help mitigate some of these issues would be to buy your own Gen4 G22, as the "standard" SF-type grip, or the optional inserts, might address your concerns about it "not fitting you" to your level of preference. Then, the armorer can still maintain, inspect, service & repair
your Glock ... and having to transition to an issued one in the event of yours being taken into property/evidence may not be such an imposition.
You're about to embark in a LE career. Don't obsess on the weapon. Lots of other critical things to consider, learn and worry about on any given day. It's a handgun ... being used as a dedicated duty weapon. That's all.
Lots of gun owner/enthusiasts who get a cop job want to think about the GUN.
How well can you DRIVE?
How well can you WRITE?
How well can you speak in PUBLIC, to small or large groups of people? How about testifying in court?
How's your level of physical fitness?
What's your general life experience?
Congrats, BTW.
Indulge your firearms enthusiast interest on your off-duty weapon, if you wish (with all the same potential issues still facing you, of course).