I don't object to that part, merely the erroneous assumption that the transwarp conduits mentioned onscreen were intended to be Borg-made. That's speculating beyond the actual evidence.
Well, we DO have evidence.
In TNG, when the ENT-D encountered renegade Borg and Lore, Geordi said the Borg established a network of TW conduits in the AQ.
Its possible these conduits are the same ones seen in use in Picard and later on in the 32nd century... just became neglected over time.
The Burn could have caused some ships to explode in transit, though to be fair, Trek interstellar powers in the 32nd century seem to be quite a bit dumb (or DUMBER) than their historical counterparts.
They already had ridiculously automated tech in the 23rd and 24th centuries... they could have sent a few automated self replicating probes which would clean up the debris from the conduits after the Burn using tractor beams and projecting low level subspace fields to lower their inertial mass for easier towing. The said drones could have been programmed to create more of themselves from the old debris (which can be used as raw material) and clean up the conduits over time... large excess leftover matter (aka broken up ship hulls etc.) should have been brought out of the conduits and towed to nearest facilities where they'd be broken down into base elements and reconstituted into new starships or into something else.
Transporters and replicators alone could easily do that... with programmable matter, it becomes even simpler - but then again, Disco is a show that completely ignored the fact that subspace fields can be used to lower an inertial mass of ships (which was already used in the 24th century to improve manueverability of large ships to the point where they moved with ridiculous ease) and went with 'detached nacelles' to do the same.
Could have used another reason for detached nacelles or whole hulls. Perhaps improved overall safety in case of catastrophic explosion in one section of the ship... upon which safety mechanisms would push detached sections away from each other with repulsor tractor beams.
None of the live action series ever went into full blown detail on what Trek technology could do with proverbial ease even after seeing tech partly being used for those purposes.
Seriously, this issue of having completely clogged up TW conduits is a bit daft. Just send the probes, leave them to work on their own and that's it. It may take a bit of time (depending on the number of conduits, etc... but since the Burn happened, SF could have easily charted cleaned and charted those conduits).
Also, I'm surprised nothing came out of Borg TW tech VOY used in the DQ. Dead TW coils could still be studied so they can be reverse engineered.
Though, I wouldn't be surprised if part of the reason why the conduits may have been left as they are was to PREVENT potential hostile ships from approaching the weakened UFP/SF HQ... although, again, with the excess matter from those ships in the conduits, SF could have easily built numerous defensive automated and cloaked turrets which would inspect ships as they come through... and if Emerald Chain ships emerge... they could be fired upon immediately and disabled on sight.
Oh well.