It's been a few years since I successfully completed a legacy following the rules perfectly, and I won't be of much help there, I'm afraid because even I question how I managed.
Though I will say that these days I choose to play my game loosely following the rules, and applying my own and find it easier to get through several generations that way. Personally I find it hard to stick with it for long if I'm too restricted by rules. So, if you find yourself getting bored maybe loosening up with the rules or tweaking them for more freedom would suit you better? Sure, it won't be the good ole legacy challenge, but it's your game and you may as well do what you need to do to enjoy the ride. Some strive with more definite rules to follow and things to achieve and it helps them get through it, for others it just doesn't and that's okay! We're all different after all.
In my current "legacy" I name each generation alphabetically (founder was Aeryn, who married and had the B generation, who then produced the C generation etc.), have strict patriarchy without player interference of gender outcome (and I swear I don't seem to be able to get a son on the first try), strict bloodline, founder started with 0 money on a big lot and so forth, no money/skill/needs cheats allowed, heirs have their typical rules of straight A's, completing aspirations and making it to the top of their careers and such, the usual shenanigans.
I do, however, play rotationally, and allow my heirs to move out of the house, and the founder's lot serves more as a legacy landmark and is where all my heirs eventually go to die as they grow old. I keep their graves, some personal belongings and portraits of them there and expand on the house with each generation, elders essentially inherit the house and do renovations and upgrades in their retirement years. I have aging off and kill for plot, or when I deem elders to have lived "long enough". I want to enjoy all the different worlds and I love building and furnishing so I just can't keep myself in one place forever.
When island living came out I wanted to experience the pack with one sim and considered just making a new save, but decided to instead use my then heir's sister Briar to do it because the pack fit her best and I got some fun plot out of it. She moved out with her future military husband who was sent off to be stationed in Sulani for peace corps purposes, and Briar was essentially a military wife. By the time I had experienced everything I wanted in terms of the pack I had fleshed out the "legacy" and had an actual story for her. She was a stay-at-home mom, producing her share of the C generation and I found myself more compelled to have my heir and the "core family" actually interact with their extended family once I switched back to them, as now that part of the family tree actually had something interesting going for them. Once I switched back, the heir Bellamy married his sister's best friend from Sulani and due to joining the military career and having the Strangerville aspiration he moved to Strangerville to raise the core C generation.
I followed the same principle with those children and the eldest daughter of the C gen is who I used to explore Realm of magic as it came out and for plot purposes had her run away from home (long story), and Discover University came out just in time for my heir to head to college, and I took him and his brother there separately (heir went to UBrite for drama and is now exploring Get Famous as an actor, and his brother attended Foxbury and made a Servo he now lives with in San Muyshuno)
Those are just examples of how I've made (a cherry picked few) legacy rules work for myself when I've found myself getting bored and quitting previous legacy attempts, besides the ones I somehow managed (seriously, how did I? I may never know). Other simmers who actually manage to get through legacies according to the rulebook (I bow to thee, you absolute legends) will be of better help in terms of the actual challenge, but I figured I'd share an alternative in case you're someone like me who needs more leniency, while benefitting from the structure and guidelines the rules provide.
TL;DR if the legacy challenge is intriguing as a concept, but you find yourself unable to stay motivated and inspired for long, consider being more forgiving and lenient with the rules and come up with your own to better fit your playstyle and what you enjoy.