I have read somewhere: Kufr (disbelief) of I‘raadh (turning away from the truth): when a person has a doubt and is able to learn and find out the truth yet he is satisfied with what he is upon and does not seek the truth.
What does it mean exactly? Does it mean that whenever a person has a doubt or is unaware of a ruling or has doubt about Kufr, he must not be lazy to find out about it, and otherwise it is Kufr? Moreover, I have done the above mentioned things, but I delayed sending you a message because I was feeling lazy. Is that Kufr too?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

You should totally ignore this Waswaas (obsessive whisperings). What you mentioned is not Kufr in any way. Rather, even if we assume that it is true, you may bear a sin for falling short in acquiring knowledge, but it does not amount to Kufr, and it is not Kufr of I‘raadh. All these thoughts are mere satanic whisperings, and their treatment is to ignore them and strive against them.

As for Kufr I‘raadh, an example of it is the reaction of the polytheists to whom the Prophet, sallallahu ‘alayhi wa sallam, was sent. They turned away from what he brought with him of signs and proofs and settled for blindly following their ancestors. Thus, they committed Kufr by turning away from the truth and their refusal to contemplate the arguments. As for the Muslim who fails to acquire the required religious knowledge, he may bear a sin for it, but this is not Kufr in any way. Therefore, ignore this Waswaas, as we advised you to do before repeatedly, as getting carried away with it leads to grave evils.

Allah knows best.