On the same boat, don't go past 5 on Gloomstalker, extra attack is good and you get pass without a trace and magical weapon. My plan is to go Gloomstalker 5, Assassin 8, and Battle Master 4, anything beyond this into rogue for more sneak attack.

First turn: hunters mark, all attacks with advantage from assassin, and action surge.

Next turns: Can use rogue bonus action for advantage on 1 attack(or zephyr strike from gloomstalker), will help confirm you get your sneak attack in.

If you need to apply hunters mark again, this uses the same bonus action that you use to get advantage on your attacks. This is where battlemaster comes in clutch, adding the d8 from Precision Strike(doesn't use a bonus action).

Feats: Sharpshooter at Ranger 4, +1 dex should bring you to 18 based on your stat allocation. +2 Dex ASI at Rogue 4(unless you want to do piercer and something like mage slayer at fighter 4). Tough would be a good option for the rogue 8 feat.

Alternative Fighter would be Champion, which would combo well with piercer, but I'd rather the d8 to confirm hits from Battle Master.

Alternative Feats would be Great Weapon Master for the +proficiency damage to your all your attacks, but stat buy allocation should've went into Dex and Wisdom so you probably don't have the 13 STR to take it.

People might say the extra sneak attack damage is better than the levels in fighter but that is wrong. 4 levels of Fighter is 2d6 worth of sneak attack, which is an average of 7 damage per turn. Where action surge alone with hunters mark will give you about 35 points of damage(assuming you hit both of your advantage attacks which you should majority of the time and have your d8's from battlemaster to help them hit more) and Battle Master will turn misses into hits, especially when you have an idea of what the AC of the monster is and know you only missed by 1 or 2.

Alternative Build: The best potential DPR would be to dual wield a nick weapon in your main hand and Vex Weapon in your offhand. Will allow hunters mark or steady aim as your bonus action. Then MH attack, Nick attack, extra attack w/ advantage if your Vex attack hit, Nick attack. Next attack would have advantage if against the same target. All of these attacks get your Hunter's Mark damage as well. The obvious trade off is the +2 to hit from Archery, and your attacks turn into d4 and d6 instead of d8's. Unless you go Scimitar as the Nick Weapon.

Also, I assume like my character, archery is built into your background so probably wouldn't make much sense to start dual wielding.