Are you already or would you consider hosting pinball parties and IFPA pinball tournaments at your place?

If so, I always felt that having 9 to 10 games is a nice size collection for tournaments.
With 9 machines and room for 10, you have at least 8 to use for many tournament formats, and 1 pin can be an alternate if needed, if one of the 8 machines breaks during tournament, and is too difficult or time-consuming to fix on the spot.
For a pingolf tournament, 9 machines works out great to use for playing a 9 hole or 18 hole tournament.

Having space for 10 pins, even if you only have 9 pins is great. This makes it a bit easier to move in a project pin to work on, or to have room to move collection pins around, or to work on pins, etc.

So, related to your situation, it appears you have 9 pins in your current collection, and in your post above you are mentioning space constraints, but want to stuff in a few more machines. Perhaps just limit your collection to 9 or 10 pins to leave enough room for moving pins around in you house. It seems like you already have a decent mix of machines (late 70's early ss thru modern 2018/19 pins). Perhaps get one cool fun e-m type to help broaden your collection.