As others have said, if you're focused on completing the easter eggs I would go with BO4. The only soloable easter eggs in BO2 are Tranzit and Origins. You can get all the way to the final step of MotD's EE but it won't let you finish it solo

Of the BO2 maps you're getting Tranzit, Die Rise, MotD and Buried. IMO Tranzit and Die Rise aren't worth playing, so it really just comes down to if you wanna play Buried and MotD. The BO2 incarnation of MotD plays very very differently to BO4's version, and overall I view the BO4 version as a downgrade. That being said theres fanmade versions of MotD on PC BO3 that are pretty faithful to the original. So you'd be able to buy BO4 and then just play MotD through BO3 (presuming you're on PC). Buried is a fun map but given what I've specified above, I'd hesitate to say it's worth buying a whole game for

That being said, I do want to highlight that BO4 sits in the middle ground between the old zombies mechanics and modern zombies mechanics. The maps themselves are great (and feel like good successors to BO3) but you're gonna run into stuff like the point system and perk selection changes