Getting higher tier troops shouldn't be easy. In Warband, it was too easy, making low-tier troops essentially pointless because they would be bypassed instantly. However, even in Bannerlord, training up recruits still isn't particularly difficult. I can't understand how you can think it's too much of a challenge. There are plenty of looters that are easy to kill, and bandits aren't too tough if you can distract their attention (and bandits shouldn't be pushovers to low-tier troops in the first place, peasants are their primary prey after all, and if you're charging recruits directly into archers you are doing something wrong) There are also a myriad of perks to level up recruits without battling at all. Take 3 perks requiring only 50 Steward and 100 Leadership and you can level up tier 1 by doing absolutely nothing except waiting 17 days. Or, you could simply pay to level them up by giving them gear, or plop them in a fief with a governor and passively generate high tier troops. And with a good medicine skill, you hardly need that many recruits in the first place and can get by with tier 2+ unless you are throwing your high-tier troops fighting in battles with bad odds.
My problem with the levelling system is not that levelling up low tier troops is too hard, it's that levelling up high-tier troops is too easy, and the fact that there is no meaningful mechanical distinction between noble vs commoner recruits, despite the fact that Taleworlds have had a myriad of mods, which have successfully implemented a bifurcated recruit system, to learn from.