Good morning @shiftrox
I found the day cap to be something that's easy to track and for everyone to understand.
I think it should be enough to deter people from multiaccounting or boting, as as you mentioned, you need 10 HP to start, so when you cap the Seed tree, bot should make a new account, power down the 10 HP, wait 3 months, move the HP, and all that for 0,6 HIVE for a month, don't think it would be worth it for them.

Personally i'd go for 30 days, by then you know if you want to keep playing or just farm the rewards.
The only way to remove the cap is if they buy 1 tree, atleast it would mean they put something back into the game, and it will take a long while to farm it back!