I've added the Mining Level 5 tag for Lootr blocks, but I'm still able to get a drop

Make it harder to break

What exactly are you trying to achieve? There should be a configuration option that prevents chests from being destroyed or broken unless done in creative mode.

This probably has to do with the fact that the chests aren't using loot tables but manual drops ... due to 1.14 code that's no longer relevant. That's a todo.

Make it harder to break, and to also dissuade them from breaking. I used CT to edit the block hardness, I would only be missing the harvestability tag compat, which Would force people to need to use a Harvest Level 5 tool to break the block in a "decent" time

If this is still an issue for you, please open a new issue. This wasn't a huge priority for me especially given the custom code already in place to prevent people from breaking containers.