That’s not a tiny nerf, it would just make monks nearly useless. Problem isn’t that they convert. It’s how fast they do and how with 0 upgrades they add a lot of value. The conversion mechanics should change. Uniform 28% probability per conversion interval is quite high. There’s 28% probability the unit is converted in 4 seconds, 28*72/100=20.16% probability its converted in 5 seconds, 14.5% in 6 seconds, 11% in 7 seconds which means 74% of the time conversion happens in under 7 seconds and only 26% of the conversions take 7-13 seconds.
My suggestions would be to first change the conversion mechanics into increasing probability as time goes instead of a uniform 28% probability for each conversion. The first CI having 10% probability, 2nd having 15%, 3rd having 20% and so on. This will change the conversion time distribution to 10%, 13.5%, 15.3%, 15.2%. So you’ll only have a 54% chance of conversion in the first 4 seconds instead of 74%. And very little insta conversions (10% instead of 28%)
Other than that there needs to changes to monk tech tree and costs. Particularly predecessors to block printing and illumination should be added. The default monk range should be 7 and its resting time 80 seconds. The predecessor to block printing grants +2 range and that to illumination gives 33% faster faith regain bringing the values to the current default. These upgrades should be 150 gold each and block printing, illumination should become 300. Fervor should be more expensive as well.