While probably not the most optimal build for DPR, it does have reaching character.
Gnomega Red
Deep Gnome Bear Barbarian 5/Battle Master Fighter 3
Take Sentinel as your free feat (I'm assuming that its part of your character creation house rules) and swap out the Barb 4 ASI for the Dual Wielder feat. TWF FS is a given for this. Dual wield a pair of whips and whip the shtako out of everyone around you.
While you're using the smallest damage dice, your flat damage bonuses would help to mitigate that. Just because a whip is a finesse weapon, that doesn't mean you can't use Str to make attacks with it. The tradeoff is that you've effectively doubled the range of your stickiness. Plus using a reach weapon helps compensate the 5' speed loss by playing a Small character, although Fast Movement does that itself too.
For maneuvers, I'd go with Precision Attack, Riposte, and either Menacing Attack or Goading Attack. Precision Attack is useful for making sure those opportunity attacks hit. Riposte gives you another option for making an attack as a reaction, on top of those provided by Sentinel. Menacing Attack and Goading Attack are both good options that add to your stickiness. If you feel up to it, a later ASI could be spent on getting Martial Adept for the other maneuver, Tripping Attack, and an extra superiority die.
As for 9th level and beyond, your stats leave yourself open to the possibility of going Swashbuckler Rogue the rest of the way to increase your DPR potential. The primary tradeoff of 12 Rogue levels would be losing Barb HD, Feral Sense, Brutal Critical and Rage progression in exchange for 6d6 Sneak Attack, more options for your bonus action and reaction, Evasion, extra skill proficiency, Expertise, one extra ASI, and Rakish Audacity. I would go with 20 16 18 10 10 12 as your stats. The tradeoff of a slightly lower initiative for more HP gets mitigated at Rogue 3, when you would get to add the +1 from Cha to initiative. While you would be more Rogue than Barbarian by class level, you would still mechanically be a raging whip micromachine. Just some food for thought.
Edit: Going Rogue 8/Barb 4 for the last 12 levels might be a happy medium. You'd get up to Evasion and 4d6 SA from the Rogue side, while making Brutal Critical your capstone feature. That's of course assuming that the campaign goes up to 20th level. If you're ending sooner than that, stick to just the Rogue levels.