In an ideal world . . . Terra Blade would be post-Golem, I think. But I see even less of a clean way to do that, aside from locking Mothron behind Golem, and I don't think I can swing that one, it would require rebalancing multiple items to account for it, and probably buffing Mothron as well. Too much damage left in its wake.
As for the whole Excalibur/True Excalibur situation, I think True Excalibur has room to grow, it could support buffs. Its biggest limitation is that if I buff it too much, it invalidates Terra Blade, which could not be similarly buffed in turn. This re-arrangement will probably allow me to buff all three.
-> Mothron spawns post-Plantera (it drops EoC and Mothron wings)
-> After you defeat Golem, Mothron now also has a chance to drop Broken Hero Swords
Maybe adding Beetle Husks, Shroomite and Spectre bars to the Terra Blade's recipe? (Only using Husks is a bit weird)
Here's an idea I had. This makes True Night's Edge post-mechanical boss(es), True Excalibur post-Plantera, and Terra Blade post-Golem.
True Night's Edge (crafting)
– Night's Edge
– 1 Soul of Fright
– 1 Soul of Might
– 10 Soul of Night
– 1 Soul of Sight
OR
True Night's Edge (crafting)
– Night's Edge
– 10 Soul of Night
– 3 (any mech boss soul)
True Excalibur (crafting)
– Excalibur
– 10 Soul of Light
– 6 Spectre Bar
Terra Blade (crafting)
– True Night's Edge
– True Excalibur
– 1 Broken Hero Sword (now post-Golem drop)
Broken Hero Sword now only drops from Mothron post-Golem (and possibly also 0.4% from Frankenstein and Swamp Thing), or drops from Golem as a guaranteed drop the first time you defeat it (similar to how the Grenade Launcher drops from Plantera as a guaranteed drop the first time you defeat it). Now that Broken Hero Sword no longer drops pre-Plantera, Mothron is moved to post-Plantera to be in line with its main drops.
All three swords get rebalances to be in line with their new places in the progression. True Excalibur now makes True Night's Edge obsolete, but both are obsoleted by Terra Blade, which ends up being on par with the Influx Waver. (The choice of which to use should come down to preference and convenience.)
So ideally you want the terra blade to be crafted w/ the broken hero sword after golem but at this point can only gate mothrom behind plantera. Would an ancient manipulator for the crafting station (with a suitable buff) be considered too late? Theres not much in the way of materials that is exclusive to post golem content besides fragments and golem loot drops.
Currently Mothron is gated behind the mechanical bosses, but most of its drops are gated behind Plantera. Broken Hero Sword can easily be gated behind Golem in a similar fashion. Therefore, no special crafting station is necessary. Naturally, Mothron would have to be moved to post-Plantera, as it would no longer have any post-mech drops.
The Broken Hero Sword can even be made a Golem drop to reduce RNG, but it's probably better that it remain on Mothron, as making it a Golem drop would reduce the usefulness of Solar Tablets and Solar Eclipses.