Greg, you are already pretty well covered in all categories, especially if you are OK with CCW full sized Glocks, otherwise the G23/G27. Most CA agencies only allow 3 guns on your permit anyway. Any of your guns would be ok for home defense, although larger frame guns are easier to manage. For training, hands down, the G17. For USPSA/IDPA/Steel Challenge, 3 gun and local action (non-sanctioned matches), The G17/G35/G21SF and G31 all fit in competition, the G23/G27 not so much, it all depends on what game and division you want to be in.
G17 USPSA Production, Limited , IDPA SSP
G35 USPSA Production, Limited, Limited 10, IDPA SSP
G21SF USPSA Production, Limited, Limited 10, IDPA SSP and CDP (download to 8)
G31 USPSA Production, Limited, Limited 10 and IDPA SSP
The 40S&W and .45acp can be downloaded to minor power floor of 125, and the .357SIG was just approved for major power floor in Limited and L10. If you put a dot on any of the 4, you can run USPSA Open and IDPA not for competition. Again, all of these divisions work for Steel Challenge and 3 gun. So it sounds to me like you just want another Glock, and if it were only based on competition, I would get another G17 Gen3. Backup guns are necessary if you are shooting once a week. I have 4 G17’s, 2 G31’s, 2 G17L’s, and my original G19 from 92. While the G17L is interesting, like the G24, it really only fits USPSA Open. Keep in mind that all full sized small frame Glocks are the same, sans ejector and extractor, uppers and lowers are interchangeable., 17/34/17L,22/35/24,31. Your G21 can also run 10mm, 40S&W, .357SIG with conversion barrels, including some interesting stuff like 9x25 and .45Super.