If you want my sincere advice how to grow from 4 subs to 100 then to 500 then 1k quickly, find abut 5-10 channels at the 5k-10k subscriber mark and closely analyse their videos. Look for any abnormal traffic and breakouts. This will reveal what is popular and caused them to breakout. Look for trends in those 5-10 channels. Research their kws with Tubebuddy and in the Yt search bar. Then make videos on similar topics. The algo will associate your channels together assuming seo and analytics line up.
I think what's popular (ie: what gets picked up and promoted by the algo) is to a large extend dependent on channel size/authority and viewer behavior. For example, viewers go to Toy Freaks in droves and herds to watch their bad baby videos, but if you put out a bad baby video, you'll get 20 views (nothing personally or professionally against you as a person, only your channel authority). However if you put out a Thomas Accidents will Happen video, you could get 5-10000 views from all the committed Thomas fans out there, and be placed as suggested in a Thomas video from a 10k sub channel. It's just how the damn thing works.
Don't pick channels greater than 10k subs, as the algo won't associate you due to built in leveling. You have much higher chance of being in suggested of a 2k-5k channel than a 50k channel.
What's popular is important, but their is a channel size effect that plays into it in a very large way.
