You cannot raise the proc rate, but using a TH value lower than the enemies, will lower your chances to proc.

Using only TH+8 to try procing to 10 is inefficient.

Using TH+22 is no better than using TH+14, as 14 is the highest value you can proc.

This is the community now telling you TH+8 is not the most effective means by which attempting to proc to TH+14.

This information is also on BGwiki, and has been:

Example 2: If you plant TH6 on the monster by pulling with your TH gear on and then swap to TH3 to kill the monster, there is a 3-level gap between your current TH level and the monster's current TH level. This makes it comparatively more difficult to proc (approximately a 2% chance per melee round).
https://www.bg-wiki.com/ffxi/Treasure_Hunter

As well as FFXIcylopedia:

The chance to increase your TH bonus greatly diminishes with each level of difference between the target's current Treasure Hunter effect and your current Treasure Hunter bonus. For example, if you apply TH8 on a monster, and then drop your TH bonus to 3 by switching equipment, further increments will be that much rarer.
https://ffxiclopedia.fandom.com/wiki/Treasure_Hunter