Bear in mind that sexual behaviours are not all hormonally driven, but many are learned behaviours. In other words, gelding an older stallion is not necessarily going to make a huge difference in disposition - he may still evidence “stallion-like behaviour”.
People also often make the mistake of believing that a gelding will not breed a mare, and it’s therefore OK to turn them out together - in fact this is often the driving force behind a decision to geld an older stallion that is “not breeding any more”. FAIL! Geldings can and do breed mares - not all geldings, but those in particular that were gelded older are more likely to. While they cannot of course get the mare pregnant, they can transmit disease/pathogens from one animal to another, and furthermore, the risk of injury to one or more animals is significant in a pasture situation.
There is also a health risk connected to gelding older stallions, with a greater risk of hemorrhage.
On the subject of frozen semen - don’t make the mistake of getting the semen frozen, having it looked at post-thaw and being told “it looks good” and then gelding the stallion (if you’re still contemplating that after my above comments!
). Thawed frozen semen may LOOK good, with excellent motility, but it could also still be completely incapable of achieving a pregnancy. Breed mares with the frozen semen and achieve a pregnancy before gelding the stallion! That way you’re not going to have a major disappointment down the road after you’ve already removed the essential equipment!
Overall, my advice to you is to think long and hard about this. It might result in a wonderful gelding; it might result in a stallion-like gelding; or it could result in disaster.
Good luck, and hope this helps.