How to selectively install new macOS features on my old and slow iMac?
The macOS installer does not provide the functionality you are seeking. With the default update process your goals are simply unachievable.

That is: my firm recommendation is: do not try this and be careful with the following.



That said, if Pacifist works with Mojave (not tested):


you might want to disable automatic updates
you might try to download a standalone updater from Apple, when it becomes available
you open the resulting installer package in Pacifist and look for all the features you want and install only those, ignoring the rest.


Looking at the size of the packages Apple distributes these days it seems quite questionable if this route is even remotely able to provide you with even the slightest advantage regarding time. The possibility to screw up even more is also higher this way. This is more a theoretical excercise along your intended plans and not really a practical solution.



The only practical solution for your plans is to let the default Apple installer run its course.  After that finishes, you disable SIP and then disable all those things you do not want. 
Only this time you do record your steps and pour them into a script, like done here:


MacOS disable services or
Disable bunch of #$!@ in Sierra (Version 2.1)


That inconveniences you just a tiny bit more compared to a regular install left at all defaults and is finished after the next update in under 10 minutes. (If Apple didn't change too much under the hood: like introducing new services your  script doesn't know about (and you do neither) or introducing new dependencies to those things you threw out.) Given the status of Mojave your old and slow iMac might benefit from staying away from this bleeding edge. Older OSes do not change so much as is dutifully expected from this newest kid on the block.

There is no safe way to do this. Backup hourly. At least. (Having a spare Mac or partition or VM to try this out might be an idea not easily dismissed as well.)