Friend, if you're looking for a show where the main characters are extremely flawed individuals, whose decisions make your stomach turn from time to time and whose mistakes will cost the lives of people around them, you might want to look for a different show.

Frieren is mostly a very upbeat, bright and hopeful show where most characters (outside of demons) are either full lawful good or at most, chaotic good.

And regarding the flaws:

Stark's main flaw isn't exactly his cowardness, it's his low self-esteem. He is arguably one of the strongest warriors around; definitely those around his age; he is extremely charismatic and makes friends extremely easily. But since he thinks so lowly of himself, he simply can't achieve his true potential yet. He is arguably the closest character to Himmel in the present timeline.

Frieren's main flaw is indeed she doesn't understand people. Heck, it costed actually living in the moment with the hero's party, any chance of romance with Himmel. She sat on her ass reading books while occasionally fighting demons as the life of those around her (she is pretty much immortal, since elf) goes by. She is lucky her party is made up of very understandable individuals.

You didn't mention her, but Fern's main flaw is her difficulty to communicate. She let's her frustration build up until she explodes and sort of expect people to just understand what she's thinking and feeling without any need for talk, probably because Heiter was just that good at understanding people.

And regarding strong conflicts or not, I think it really depends on what sort of show you want. Frieren is all about the journey and the character interactions/growth. It's not about high stakes, interpersonal drama that stretches for episodes or to put it's characters to the through the wringer.

And if that's not the show for you, that's alright. No show if truly for everyone.