I created an engine for the purpose of adding your own custom blocks to Minecraft Datapacks.

More info (How it works and How to use it) right here (a generator for creating blockstate and model files can be found here)
By default, the pack comes equipped to add around 54 new blocks. However, this can be expanded very easily by installing this datapack, which turns most Doubleslabs into unused blockstates, which you can then use in this pack.

This pack isn't perfect, but it's very lag-friendly. This uses absolutely zero entities for the custom blocks. Not one. Zilch. Zip. No entities. no markers, no displays, nothing.
There are a few minor bugs but they mostly do not present themselves.

The download contains 3 custom blocks for you to use as reference.

This is only an engine for adding the blocks, you will have to handle making them craftable/obtainable.

This is an ENGINE, rather than a LIBRARY. This means you directly work inside of and modify the datapack files, rather than using it with another pack. You can technically use this as a library, but I'd only recommend that to advanced users who know what they're doing.

Anyway, I am once again cursed by not knowing what category to put this pack under. I'll just hope I get lucky and enough people see this :)

Featured in Spruced Vanilla(?)
Thanks to joshieman for the Title in the Thumbnail