#Your best bet is probably a Monster Slayer using a Hand Crossbow, with the Crossbow Expert and Sharpshooter feats

I'll start by pointing out that I don't think that "assuming attacks hit" is an especially effective way to gauge character damage output. Many methods of producing damage, like the Sharpshooter and Great Weapon Master feats, specifically exploit the fact that using them causes you to hit less often, dealing more damage when you do hit. So ignoring that context will cause misleading results.

In my table below, I've included calculated average damage-per-round for various Armor Class values. I've included the hypothetical "AC 0" to simulate a character "basically always landing their hits", but I don't recommend solely relying on it, because it simply does not represent normal gameplay.

Additionally, these values are averaged across all rounds of combat, but you can get to 5 rounds of combat by simply multiplying each value by 5, since there's no confounding gameplay mechanics that would make that inaccurate.

Below are the important stats for each character I've considered:

• Horizon Walker Heavy Crossbow: takes the Archery Fighting Style, and the Crossbow Expert feat, for an Attack Bonus of +9, Damage Bonus of +4. Always uses their Bonus Action to invoke their Planar Warrior feature.
• Horizon Walker Sharpshooter: same as above, but also takes the Sharpshooter feat and uses it relentlessly. Attack Bonus +3, Damage Bonus +13.
• Monster Slayer Hand Crossbow: Archery Fighting Style, Crossbow Expert feat. Attack Bonus +9, Damage Bonus +4, uses Bonus Action to make another Crossbow shot. Slayer's Prey persists over multiple rounds, so it's been included, and we presume it is applied before combat even began (because there's no mechanical restriction against it)
• Monster Slayer Sharpshooter: Same as above, but takes the Sharpshooter feat. Attack Bonus +3, Damage Bonus +13.
• Monster Slayer Glaive PAM: Assuming a different race to get optimal Strength stat, takes Polearm Mastery feat. Attack Bonus +7, Damage Bonus +4. Bonus Action used to make PAM 1d4 attack. Slayer's Prey applied before combat.
• Monster Slayer PAM GWM: Same as above, but takes Great Weapon Master feat and uses it. Attack Bonus +1, Damage Bonus +13,

\begin{array}{|l|r|r|r|r|r|r|r|} \hline \text{Name} & \text{AC 0} & \text{AC 11} & \text{AC 13} & \text{AC 15} & \text{AC 17} & \text{AC 20} & \text{AC 25}\\ \hline \text{HW Heavy Crossbow} & 23.325 & 23.325 & 21.358 & 19.300 & 17.153 & 13.763 & 7.663\\ \hline \text{HW Sharpshooter} & 40.425 & 28.853 & 24.815 & 20.688 & 16.470 & 9.975 & 3.278\\ \hline \text{MS Hand Crossbow} & 25.584 & 25.584 & 23.343 & 21.075 & 18.758 & 15.144 & 8.578\\ \hline \text{MS Sharpshooter} & 51.234 & 36.308 & 31.220 & 26.042 & 20.752 & 12.560 & 3.998\\ \hline \text{MS Glaive PAM} & 28.584 & 26.043 & 23.475 & 20.858 & 18.170 & 13.962 & 6.301\\ \hline \text{MS PAM GWM} & 54.234 & 33.020 & 27.542 & 21.952 & 16.228 & 7.348 & 4.298\\ \hline \end{array}

###Observations

The first thing to notice is that by a modest margin, the Monster Slayer Polearm Master + Great Weapon Master does offer the highest theoretical damage, at 54.234 DPR against a creature where only a natural 1 misses. This superiority vanishes once we're dealing with more realistic Armor Classes, though, and the Hand Crossbow + Crossbow Expert + Sharpshooter reigns supreme, offering its own 3 attacks with the Archery Fighting Style's improved attack bonus making up for the lower damage dice.

I've included Horizon Walker just as a frame of reference, but it's never competitive with the options presented here except against especially tough targets, where the once-per-turn Planar Warrior feature starts to win out against the Slayer's Prey feature—and even then still doesn't compete against the Crossbow without Sharpshooter. Most of the Horizon Walker's most powerful features (Haste, Distant Strike) don't come online until levels 9 and 11, so by the restrictions on this build, we can't use those.

The table above was generated via the use of a tool I created. I'll step through the process for one cell, but the same process can be applied to any combination I've described. We'll use the Hand Crossbow with Sharpshooter against a target with AC 13 as an example.

To start with, we need the miss, hit, and crit chances. With an attack bonus of +3, this means the natural d20 needs to roll a 10 or higher to hit. In total this means our odds are

\begin{array}{l|l|l} \text{Miss} & \text{Hit} & \text{Crit} \\ \hline \text{45%} & \text{50%} & \text{5%} \\ \end{array}

A hit with this weapon deals 1d6+13 damage, and a critical hit deals 2d6+13. In addition, once per turn we can apply the bonus damage from Slayer's Prey. This will always (per the ability) apply to the first successful hit; and if the first hit is a critical hit, that damage die will be doubled, so we'll need to rig up some math to figure out how to apply that. We'll get to that later.

For now, we have a 45% x 0 + 50% x 16.5 + 5% x 20 = 8.25 + 1 = 9.25 expected damage per hit. Over three attacks, this amounts to 27.75 expected damage in a round.

The odds that the first hit is a critical hit is 5% (if it was the first attack), 5% x 45% for the second attack, and 5% x 45% x 45% for the third attack, bringing the total to 8.2625%. The odds that the first hit are a normal hit are 50% + (45% x 50%) + (45% x 45% x 50%), or 50% + (45% x 50%) + (45% x 45% x 50%), or about 82.625%.

So the damage dealt by the Slayer's Prey feature will be 8.2625% x 7 + 82.625% x 3.5, adding up to 3.47025 expected damage.

27.75 + 3.47025 = 31.22025, which was rounded in the above table to 31.22025.