He actually has a video about Borg assimilation that would be appropriate for this (with skippable graphics if necessary). It's used to explain things like why Amaliens allow attacks with significant collateral damage if the alternative is the assimilation of a planet, or why it's a standard feature of battleships to have self-destruct mechanisms. Some planets also use it for recruitment though the Amalien Alliance has no official policy on that.
CW: Horrifying things including body horror and such. It really is quite bad.
The video is a mini-documentary about Borg assimilation.
The process is a fairly automated form of extremely invasive full body surgery, with absolutely no attention paid to anesthetics or comfort. One portion towards the end of it seems to include the opposite - when Borg test out the host's nervous systems after coming online various nerves have the full spectrum of their pain receptors tested. The video includes some diagrams, and also realistic CGI reconstructions of the process that Lucien can play if Isabella would be okay seeing. There's a small amount of actual footage as well that she can see.
Mostly the life of the host isn't as physically agonizing as those first few days. Until brain death (typically occurring within a year due the fact that the Borg put minimal effort into keeping host's brain alive) they will be conscious of their body being puppeted as they act as a Borg. It's believed by Amaliens that the psychological harm during this period is comparable to the physical. The Borg have a habit of using hosts as foot soldiers against the communities they originate from, likely because the Borg noticed the efficiency improvements brought about by the demoralizing effect this has on defenders. There's footage of this but Lucien does not think Isabella needs to see it and it's ... something he feels bad about showing.
It's believed that most of the decisions made around assimilation are driven by the fact that they are cheaper and more effective than alternatives as well as the benefits to continued future Borg endeavors due to demoralized non-Borg.
The process is sometimes reversible if an assimilated is captured before brain death, though there have only been a handful of successes. This bit of the video is very clear on how this isn't a thing people should count on happening - it's way too expensive to scale and doesn't always work and no survivor is really okay afterwards.
Lucien will back off and pause things if Isabella seems to be having an acutely bad time with any of the video.