Hello, welcome to the community!
If you think that 40 items per day would be a good pace for you, then that option is available if you buy the lifetime subscription (unless something has changed?)
At the beginning, for me as well, I wanted more than 20 items per day, but as time has gone on, 20 items a day has actually become a really good pace for me. I am extremely motivated; I would have the capacity to study more items, but the pace of 20 items per day has matched up really well with my study outside of HanziHero. I’ve been finding that as I study through a textbook, do outside reading, and get comprehensible input, I come across a word or character that I don’t know, and usually pretty soon it comes up in HanziHero. Especially so if you’re following the HSK standards! Experiencing these things in context has really helped a lot of things stick in my mind. I could certainly memorize vocabulary quicker than this, but I find my comprehension and my vocabulary are leveling up at the same pace, so if I did more items, there would be a mismatch.
Also, in the beginning, there are a lot more sounds and components as you’re just starting out. As time goes on and you have a big catalog of sounds and components under your belt, you start getting a much higher percentage of words and characters instead.
Just wanted to offer my perspective! I’m not trying to say you couldn’t handle it or anything like that, just saying if you keep going with the 20 per day, you might find it a good pace for you. I wish you good luck with your studying!
(And if you reaaaaally want to do more than 20 items per day, you’re able to do your 20 items, change your time zone so it’s the next day, and then do the 20 items again, and then change your time zone back, and boom, you just got 40 items in one day. You didn’t hear this from me, though 
)