Thanks, I really appreciate the SMEs commenting here. I'm learning a lot.

Definitely learnt it the hard way this time. You're right that buried cables should be exact in length and fastened to a patch panel. I'll probably look into better conduit design as well for the next time (in 15 years?). Having shared conduits means I would risk damaging other cables if I tried to pull a new cable through.

Good conduit and patch panel design is definitely key for a happy life. Leaving some extra space/capacity initially is also a good idea, especially since (unless you're covering truly great distances) there's not exactly a lot of innovation in the single mode fibre space: strands you put in today (even if it's 'the cheapest stuff your vendor sells most of', which is generally my philosophy for selecting cables) will still be viable a few years down the road.

Sharing/in-place-repurposing conduit is not something I'd recommend, but if you must, leave a few dummy cables (a.k.a. 'pieces of string') on the initial install...