Oh, my idea is to make their bonus action "You are my prey" feature be the Search action, with a special thing where a successful check lets you add a flat bonus to your attack rolls against the target. Consistency over damage, and then have the smite-like spells be the main damage boost option, but with the downside of using up resources.

Tbf, Feral Senses is also pretty much an optional Warlock feature, arguably Warlock's one is better and at a lower level. Truesight is better than Blindsight unless you us the Fog Cloud spell or get Blinded often.

Half-caster Warlock is a scale model of the martial-caster divide. They could put almost any level 18 or below feature as an Invocation and people would probably say it's worse than Mystic Arcanum at that level.

As to Hunter’s Mark damage, it’s higher damage at level 5, when you miss once. Which means that the damage floor is higher in general starting at level 2 spellslots (character level 5), and the damage ceiling is lower only if you are able to make 3 attacks. At level 3 spellslots (character level 9) and onward, it’s simply the same or higher damage, excluding gimmicky action surge builds.

Hunter's Mark damage scales every other spell level, so it goes to 2d6 at level 9 if you use your highest spell slot at the time. No point in casting at 2nd level at all.

These were two different playtests, with different classes.

Ah, my mistake. The way you compared them made it seem more like they were in the same playtest.

But even in the encounter where I had 60 flight speed dragon wyrmlings, the situation constantly played out that this ranger was never grouped up with the rest of the party, so I couldn’t hit them in the breath attacks.

I promise you, "Don't group up when you fight things with breath weapons" is not a strategy unique to Rangers. That's just something kind of busted with ranged builds in general. I've been in a ton of games where the party saw a monster use a breath weapon and all immediately agreed "Split up, and if we have to be in melee at least flank so it needs to take an opportunity attack to get us both." I also just recently ran an encounter with a dragon where the Bard got out and took no damage at all, but that's on me for letting them hide around the corner and not targeting them earlier.

Rogues are resourceless utility/skirmishers. They don't need to be amazing at damage. I could see maybe an extra d6 at levels 5 and 11 to help them catch up and lessen the impact of Cunning Strikes on their damage slightly, but them not being a strictly great damage dealer is a good thing.

If the Ranger was optimized for being fast, and the Rogue was constantly tripping the enemies, why are you surprised it was hard to hit the Ranger? Also, Hunter's Mark makes them focus on single target damage, so of course they should be great at that. Hunter's Mark feeling better is a very iffy statement. The damage is more reliable when upcast to 3rd-level, but it's debatable if it's even worth upcasting over using a different spell unless you're fighting a BBEG and your whole plan is "Hit it really hard".