I've shot G21's since I was pretty much old enough to buy one.  I've always been a .45 acp girl, what can I say.

That said, and as most of you know the grip is a big brick.  I have medium sized hands and have to put Hogue grips on mine to manage a good grip.  Ironically, they actually make the grip bigger but are far easier for me to use (same thing with my Beretta 92A1).  

Lately, for about the hundredth time, I'm considering having the grip reprofiled/shaved to make it a bit smaller and maybe improve the angle (not sure if they do that).

Mines a gen 3 G21 but your experience with doing this with any Glock should be relevant.  I'm going to ask this in Glock area as well but I was curious if any of you guys had done it.

I haven't but have you looked into Lone Wolf frames?
Large Timberwolf frame

This won't be something I do myself.  I'd mess it up for sure.

Those are really nice but a little too pricey for me once it's all done.

I've done a couple where I filled the grip and then sanded it down to take the hump out. I think mac defense heats it and smashes it in though. His work is pretty nice. A few people I work with have his stuff

Try your Glock 21 slide on a Polymer 80 PF 45 frame.

I think I've decided to just keep it as is for the time being and start looking at selling it then buying a Gen 4 21SF to replace it.

Another option for you might be to look for an OEM Glock 21 SF frame, though finding one might prove difficult at the moment.

The 21 SF frame will be similar in size to a Gen 4 21 with no backstraps installed.

Most of the parts from your Gen 3 will swap over to the 21 SF frame, though the trigger housing is different. You could also look for a complete frame, with all the parts installed so you wouldn't have to worry about parts swapping.

That would be a good option but like you said, finding one is the problem.  Especially these days when everything is both hard to find as well as overpriced.  If one makes itself available reasonably I would definitely consider it.  

This is an option

https://www.armorally.com/shop/polymer80-pf45-pistol-frame-kit/

Yeah, it's a tricky situation, all the way around. Keep in mind that a Glock 20 SF frame would work also, as they are the same frame. Also gonna be very hard to find now, but it adds to your options a bit.

M&P .45 compact has a more manageable grip profile for double stack .45.

I’ve had both, the G21 is definitely clunkier.