vs depth 25 trojan (piercing weak)
normal, charge

Strike Needle level 10 (VH bonus)
137 281

Blitz Needle level 5 (VH bonus)
115 243

Blitz Needle level 6 (VH bonus)
121 254

Blitz Needle level 7 (VH bonus)
127 264

Blitz Needle level 8 (VH bonus)
133 274

Blitz Needle level 9 (VH bonus)
140 286

Blitz Needle level 10 (VH bonus)
147 298

vs depth 26 trojan (piercing weak)
normal, charge

Strike Needle level 10 (VH bonus)
137 284

Blitz Needle level 5 (VH bonus and damage pickup) I didn't notice a damage pickup but I must have gotten one
154 314

Blitz Needle level 6 (VH bonus)
123 260
Blitz Needle level 7 (VH bonus)
129 270

Blitz Needle level 8 (VH bonus)
135 281

Blitz Needle level 9 (VH bonus)
142 293

Blitz Needle level 10 (VH bonus)
149 305

vs depth 27 trojan (piercing weak)
normal, charge

Strike Needle level 10 (VH bonus)
137 286

Blitz Needle level 5 (VH bonus)
120 259

Blitz Needle level 6 (VH bonus)
127 270

Blitz Needle level 7 (VH bonus)
133 281

Blitz Needle level 8 (VH bonus)
140 292

Blitz Needle level 9 (VH bonus)
147 304

Blitz Needle level 10 (VH bonus)
155 317

Wow, that's a lot of data. O_O

But the way that weapons work with higher * ones doing worse makes no sense, if you want to keep higher gear weapons from being OP in lower tiers just make them less effective, but more so than their lower * versions. A 5* weapon at any heat level should always do at least a little bit more damage than a 4* at level 10--and at any level in the clockworks.

I think these are some general guidelines.

5* weapons are better in tier 1 and 2.
5* weapons at level 8 do less base damage at depths 19 - 26. They do more at depth 27 and 28. #1
5* weapons at level 9 do less base damage at depths 19 - 23 #2. They do more at depths 24- 28. #1
5* weapons at level 10 do more base damage at every depth.
Some 5* weapons gain more than others like better charges and more ricochets which make them favor better

#1. Acheron is an exception and will need less levels to achieve this effectiveness due to buffed base damage)
#2 Depth 23 is basil or punching bags and not really relevant except if there is a mission that has that level

Sword specific:

You could try listing specific exceptions and what level you think they should be (and maybe why). It is less concise but should be more helpful.

I am unsure about acheron but at least level 6 or 7 and lean towards 7).
I recommend the rest of the brandish line heated to at least level 7 and lean towards 8 (except obsidian blade, if you really want to gain poison on the charge).

Level 9 upgrade (maybe level 8 if you do lots of tier 1 and 2 but they are easier tiers):

Every sword except divine avenger (only when using the charge since you fire 3 projectiles instead of 1 so not sure when to recommend).
and possibly the troika line (gains statuses but probably not worth to upgrade before now)

Edit: Keep in mind that these are minimum levels recommended to not suck and not necessarily be better.

Level 9 is the break even point of tier 3 base damage since strata 5 will do less and strata 6 will do more. Level 8 is the break even point for base damage in FSC since the first two depths will do less and the last two more. This doesn't consider extras the weapon might gain by going 5* like better charges, increased range, or more ricochets nor does it consider the missing ctr low. It really isn't worth it for most weapons without extras unless you can get it to level 10 (Edit: This last sentence doesn't consider tier 1 and 2 effectiveness.).

P.S. I probably won't have time to run FSC and collect the level 10 data for at least a few days.

My general guidelines rely on a lot of assumptions.

Depth 23 doesn't behave as strata 6 so I guessed that it is the end of strata 5 (or at least behaves like it is in between). I recorded the data of depth 23 and made a leap that strata 5 handled the weird damage calculations the same way as strata 6 (improving better at the highest depth and working back). I also have to make a leap that other 5* weapons (that are not buffed like acheron) have their base damage behave like blitz with only the combuster punching bag data to back it up. I really didn't have the patience to record depths 19-22.

The last post was mostly directed at your comment about level 8 being the break even point, Bopp. I definitely agree that strata 6 is more important than strata 5.

With blitz, you don't gain extra damage even against Vana until level 8. Elemental brandishes are debatable, but I still recommend at least level 8 especially if it is glacius or voltedge since it less likely to have the extra charge hit.