You should NOT delete the families from the clipboard. NRaas has a much safer way to remove them if you really feel you need to. I find that just not playing them after moving them off houseboats is usually enough. However, the pregnant sims can cause problems as they try to change into work clothes while pregnant. So, you could use Master Controller to fire them from their jobs.
The problem with deleting the coded-in families is that the game with try to spawn some actions with them. So, like a sim who can't route somewhere, the game exhausts a lot effort and is always pulling more RAM to do something with the missing sim(s).
If you have the latest NRaas mods, then that's as good as it gets for taxi boats. However, you could give every family their own car(s) and boat(s) to help out will the auto spawning vehicles.
There will always be hiccups and some lag as so many sims are trying to do different things at the same time. Eventually, some of the sims will migrate and you'll have fewer sims pulling at your RAM.
You can periodically reset the whole town. Some people like to do that every week during the late night when fewer sims would be out and about doing things.
Also, I play with the seasonal elements turned off, so I don't get snow lag. I just turn on the snow for snowflake day. That way my sims can enjoy playing the snow one day, but I don't have the overwhelming amount of snow always falling.
I also turn off all the animal and occult options. I can still play with any that are already around or that I add, but I won't have NPCs doing a lot of magic or horses spawning where they can't maneuver.
Check any fenced-in areas and see if they might have sims and animals spawning where they can't get out. Some spots aren't problem areas all the time, but can be. If you ever sent your sims to work at the science building in Sunset Valley, you might have been surprised that they missing work. Zooming in their shows they disembarked into the bushes out front and got stuck there. It happens for field trips there too. So, I delete the bushes there and have no more problems there.
It could be I missed a spot that becomes a problem later, particularly for the large family with the toddler in a small house where the mom is pregnant. Who knows where another a baby might spawn?!
So, after Overwatch runs the first time and cleans up everything, you might still need to fix things now and then.
Also, just a possibility, but maybe the sims you added in have high poly hair that take a lot of time to render. A lot of high poly hairs together in one spot could also cause lag. I try to only play up to three sims with hair that has a high poly count or large file size (4MB or more).