My personal experience with Aholster is limited to their pocket holster, which I currently believe to be the best pocket holster available without exception. They make an IWB holster, which I think is worth serious consideration for the intended use.
https://aholster.com/iwb-aholster/
The belt attachments include:
A belt loop, which unfortunately appears to be a rigid loop. This may or may not work well depending on the design. My experience with other rigid IWB belt loops is that they tend to crack.
A J-hook, which will likely work very well with a pistol as lightweight as a 21A. I do not like them for heavier guns because the pants waistband rather than the belt bears the weight of the gun.
An "aggressive" clip. I have no experience with this belt clip. While many here seem to like certain clips, my experiecne with belt clips in general is that they are almost always either too insecure or too secure. In the latter case, they are more difficult to put on and take off than threading a belt through belt loops, and potentially damaging to clothing and belts.
While I may seem a bit down on the belt attachments offered, my really good experience with the pocket holster leads me to believe this holster is worth a try.