I was able to make it to generation of 5 of this last legacy before getting bored, and I think that's been my longest streak--and I've been trying legacy style challenges since Sims 2. So--I get it! I think for me it has less to do with the gameplay or the sims but that I'm very much a perfectionist and I always notice something about the game I'm playing that I could have done differently. For example, I let this last save that contains my legacy get too bloated with townies, I now know a better way to manage them.
What worked for me to at least keep me to legacy 5 was to do a mix of rotational and legacy. Each generation had a few branches and I followed them in a regular rotation. When that got a little boring, I also started folding in some family friends, etc.
For this new save, I'm challenging myself to only play premades and create lore and relationships between all of them. The only new households will be for scenarios, and once the scenario is complete I won't play them anymore. The goal is to create an interesting, fleshed out "base save" that I can then back up and boot up whenever I finally decide to restart my legacy.