Looking for advice on Coop + Run size for my situation (10-15 part-time free-range birds)

[3KillerBs]

The nest boxes can be pulled out to allow for more floor space. I plan on pulling at least half of them.

5x8' coop with 8x4' run - builder/seller says this is good for 10-15 birds:

Even a full 5x8 would still be only 2/3 of the space you ought to have for a 15-bird flock.

Unfortunately, manufacturers wildly exaggerate the numbers their coops will hold, basing their claims on legal minimums for commercially-kept birds rather than sound, backyard-style management.

Additionally, that coop has minimal ventilation and what ventilation it has is in the worst possible place -- right next to the roosts.

If building isn't something you feel comfortable with, how about a shed conversion instead? You'd get a lot more space for the money.

https://www.backyardchickens.com/articles/the-mulligan.74743/
https://www.backyardchickens.com/articles/aarts-coop-page.65912/
https://www.backyardchickens.com/articles/the-scoop-on-a-rubbermaid-big-max-coop.76444/
https://www.backyardchickens.com/articles/toy-shed-conversion.64879/