Good Call out I would like to minimize the number of ASI/Feats lost.Remember, your gaining of ASIs/Feats is dependent upon hitting those lvs in a specific class, not in-total.
So if you go Monk 6/Ranger 2 you're still 2 lvs away from that ASI/Feat.
First, keep in mind that your campaign may not run long enough for you to see the topmost tiers of play. Most campaigns don't. I'm not saying you shouldn't give any thought to the final tier, but don't put too much weight on it.
Second, if you plan to go assassin, Gloom Stalker ranger is definitely the way to go. The synergy there is fantastic. If you can get surprise on your enemies and drop a hunter's mark on the first round, you can unload a barrage of attacks with bonus damage dice and then double everything.
Even without that, the Gloom Stalker has a lot of very nice toys, especially Umbral Sight, which makes you a god when fighting in darkness. (Do note that this requires the rest of your party to have darkvision or they will be fighting blind; if they don't have it naturally, there are spells and magic items to supply it for them.)
I'm AFB but it sounds like you are assigning too much to the first few levels of Ranger. My pre-subclass Ranger was not doing all of that cool stuff.
Thank you for explaining your thought more fully (and defining the options); the earlier post sounded like you were using everything listed.The Ranger is a class that is heavily loaded up front
Phase 2: Ranger 3 levels
Offensive Benefits:
-- Sharpshooter <feat taken at 4th level monk>
-- Archery (+2 Att roll) <fighting style>
-- Spell casting 3 spells per long rest <you only know 3 at 3rd level. I would choose from the below>
- - Hunters mark <This is a must>
- - Jump <This just fits with the build. Not needed>
- - Hail of thorns <Awesome ranged attack when used with sharpshooter for -5, +10 to the main target and extra damage to nearby CR>
- - Ensnaring strike <Good for slowing down a caster (i.e. STR save)>
Then we move to the Archetype at 3rd level:
[Alert feat] +5 initiative <but as ccs Pointed out more levels in a particular class are needed to gain this>
[Gloomstarker] +10 MV round 1 <This plus Alert, when you finally get it will be great on round 1>
[Gloomstarker] +1 ATT round 1
Kind of off-topic, but I keep daydreaming about Assassin 17, Grave Domain Cleric 2, with Purple Worm Poison. When the stars align and you use your Channel Divinity then get a surprise attack and your target fails its save* you do: (10d6 * 2) + 12d6 + 5) * 2 * 2 = 468 (avg) 788 (max). In one hit.
It would be worth playing the entire campaign just to have this work once against a really high level boss (who doesn't have Legendary Resistance, anyway).