Interesting. I’m actually heartily in favor of the “completionist” mindset. I adopted it from playing videogames. With videogames at times I would end up hopping from game to game to game, never really finishing many of them. And in a weird sense it felt less fun and it definitely felt less worthwhile. I couldn’t have more than surface level conversations about the content and I never felt like my opinions were holistic enough of that content either.
Now I only play one game at a time and I really try to finish it unless it straight up is just not fun for me. I’ve slogged through parts of games where I’ve become bored after X number of hours and ultimately been much happier for it as I witnessed everything the game had to offer (especially with regards to story). Final Fantasy XV comes to mind here. The gameplay had become tired after 30 hours or whatever but getting the whole story arc (for all its flaws) was extremely worth it.
I’ve carried this over to books. Unless I’m really not interested I will stick with the slow parts and generally I’ve been happy with the results. The Stand by Stephen King comes to mind here. It’s like 1,200 pages of tiny font and is pretty slow in the middle but has some stupendous moments and is really an epic tale I’m happy to have under my belt. I do have to pair this with a conscious “required” reading time, e.g. an hour before bed so that I’m always hitting a minimum rate so I don’t slow to a snails-pace and read a book a quarter or something.