I completely agree with
@Dabos37, I also find getting tier 4-5-6 troops too easy. This is probably due to my 1000+ hours of combined playtime in Warband & Bannerlord, as well as my knowledge rather than troops being too easy to upgrade though, so I don't think that levelling high tier troops should be made harder. But, getting them most certainly should not be made easier in my opinion.
The survival rate for high tier troops even with a low medicine level of 50 is really good, so once you get over the hurdle of levelling up a few troops to the max tier, you will continue to accumulate more than enough high tier troops. This is especially true if you level up leadership to get the perks to recruit prisoners faster.