The changes on Enchantment - Devourer are going in the wrong direction
In the patch notes explanation about
changes, you textually wrote
Kind of reminds us of launch Feral Flare. And nobody wants that
We all agree about your intentions. But the problem is that, with thie 4.21 change, instead of addressing the power balance concern of the item and improving both on jungle item balance and FF-like mechanic, you make the latest problem even worse and Devourer mechanic even more similar to the old FF, without neither solving Devourer power problem
I think that the way to solve both balance and mechanics concerns on the item is to hit hard AS and revert the nerf on base damage
20 or 25% AS(which can be later tweaked if the power turns out be too low - 20% - /too high - 25% -)
40 magic damage on hit
These would be the appropriate changes to this enchantment, in my opinion
Here is the reasoning behind my suggestion
Old Feral Flare mechanic leaded to toxic interations both for the ally team(because the champs that picked it had to remain in the jungle for long time to stack it and so they wouldn't help their mates) and for the enemy team(because they suddenly saw an enemy coming out from the jungle with a huge amount of power once FF was stacked). When you gave to Enchantment - Devourer 40 magic damage on-hit, you did it to avoid that junglers could remain too long in the jungle to stack the item, so that they could start using its damage to gank in lane and help mates right off the bat once you upgraded the tier 2 jungle item to it. Reducing the starting magic damage on-hit, just bring back the mechanic of the junglers that must remain in the jungle to stack the item to meaningful level of on-hit damage, exactly the thing you tried to avoid at first stance
And, as said above, the power unbalance of the item hasn't been solved - not even with the very harsh Warwick nerfs - , because the item gives too high AS to the AA based junglers like the Udyrs, Yis, the same Warwick(even after the nerfs), giving them too high DPS with 1 item thanks to the very high AS, and reducing the niche for currently existing items tems that provide similar stats in the game (
got completely outclassed on the offensive part, for example)
EDIT: Edited to facilitate the reading
and 
, can deal 8% of the target's current health as physical damage and 25 + stacks (after 4.21 patch) magic damage on every autoattack, all of this attacking almost twice as faster than their base attack speed (the two item combined give + 90% AS)
The interaction between them is way too powerful even for its original purposes.
mechanism?
Is also another mechanic that could work as it has a intrinsic base value that builds over time. However it caps so it is a lot easier to balance.