As for high level Ranger/monks losing juicy high level features I think it’s a rare player that will miss those features all that much next to the synergy of the build over an entire campaign.
I don't see many good high level ranger abilities.
Their best spells are level 2, which is ranger 5 (plus multiattack). Plant Growth and revivify as are good at 3rd, but all the damage spells get out scaled by being half caster.
6: Roving is nice. But rogue 2 is nicer.
7: depends on the subclass, but could be good.
9: Expetise can be gained with rogue 1, and 1d6 damage.
Maybe ranger 9 for plant growth and revivify. But also maybe druid 5 for plant growth and a lot more slots.
10: 34 Temp HP is about as much as fighter 1 would give, plus a fighting style (blindsight). Exhaustion doesn't come up.
14: 2 turn invisibly is not bad, but rogue 2 is better.
17: advantage is good, but doesn't stack with many other methods that came on earlier.
18: can pick up blindsight with a fighter 1 dip.
20: garbage.
So yea.
Ranger 5/ rogue 2 / fighter 1/druid 5 gets most of the high level ranger abilities, 7 levels earlier.
Maybe ranger 7-8, depending on subclass.