You're going to fail a lot. A moderate save on a level 16 creature is +28, so lets look at a DC 38 as a sample difficulty.
You're going to have +16 (level) +8 (legendary), -1 (STR), +2 (ABP item bonus) = 25. So that's a 13 on the dice to succeed against an on-level, moderate save. If you get something above level with a good save, that goes up real fast. ie: a creature 17 with a good save is DC 42 so now you need a 17, and I'm just assuming you never try this against an extreme DC.
Critical failure is actually a real threat for you, which is something to be aware of because it can be not great. In the first case you crit fail on a 3 and in the second case you crit fail on a 7 (this is more likely than success is).
With a +4 STR, that 13 changes to an 8 and the 17 becomes a 12.
I also don't recommend taking the Halfling ancestry penalty if you want to do this. Use the alternate ancestry boosts instead.
Either that, or get assurance and accept you'll only be able to do it against weaker enemies, but you'll always succeed at that.
What they said, but also you'd be putting your PC (a more vulnerable one than theirs) in harm's way or even wasting actions moving forward, not to mention how mobile a lot of enemies will be. Helping your warriors mobility will often help more than a trip would.
Look at Telekinetic Maneuvers where all the numbers naturally scale w/o such risks or investment (except Cha of course). Check with your GM that your size doesn't alter who the spell works on (as technically the size limitations apply though intuitively they shouldn't). You could pick up MCD Sorcerer to get more 2nd level slots (and damaging Focus spells, w/ Blinding Beauty or Horrific Visage for debuffs).
There's also Falling Sky, Telekinetic Bombardment, Heightened Sleep, and other Occult spells that can make enemies prone. Or if it's just to debuff for your buddies, there's the famous Synesthesia where even getting 1 round is a 50% damage boost for your warrior buddies (and other allies attacking AC or Ref saves).
I also don't think you can be effective at tripping if you leave your STR so low. You are going to need to buff STR at least a bit. It doesn't have to be maxed though.
Halfling has a penalty to their STR attribute. That doesn't mean that your final attribute bonus is going to be -1.
At the very least, they could use the alternate ability scores and get two chosen boosts with no penalties.
Then put a boost or two into STR at level 1 and boost it again at each level up that gives ability boosts. That will end up with a +4 STR by level 10 or 15.
Since the campaign is starting at level 16, that should be fine.
Put some skill boosts into Athletics and the whip bard will only be a point or two behind a barbarian at being able to trip things.
Interesting, so essentially I need Str that's the only way to make it work.
The most effective way is to just use the Telekinetic Maneuver spell on high value targets.
Although there are probably more effective spells that you could cast first (Synesthesia ) but it's not bad as a backup.
Especially because Telekinetic Maneuver is a 2nd level spell, as you level those spell slots are less needed. You can also buy lots of scrolls of it as backup. At level 16, cheap consumables like scrolls give you a lot of staying power.
As a bard, start of combat with your preferred song, synesthesia on a high value target (if any), and then you can start casting from scrolls to generally debuff appropriate enemies. Remember, spell DCs/attacks are all the same even when cast from scrolls.
You could also consider playing a fighter with the bard dedication (from Free Archetype). That would give you a great platform to do maneuvers and give you access to spells for buffing your party. And even certain spells (again Synesthesia) can have powerful debuffs even on a failed save.