He who fights with monsters. Dragon heart series(might not even really be considered litrpg due to the type of UI.) Magnus. Crashed into Fantasy( very minor spoiler all have system but mc has a bunch of other classes others don't plus things others can't do.) Blessed time? (Early to mid book spoiler <!most get a UI upon turning 16 but due to circumstances mc is able to get it earlier!> ) Exalted Warlock(similar UI as Dragon heart series). Memento Mori: Death Incarnate (technically through a grimoire and not a UI) Rise of the Night Stalkers(mc and a few others [like 2ish] have system) Incursions( end of book spoiler <!find out that at the end mc is not the only one. !> )

Hope these suggestions and minor breakdowns for those that may be didn't fit exactly what you want help and that you find good books that you like!