in order for a sin to be a sin of omission one must choose to not act
Yes, that’s right. A sin of omission is where we fail to do something we know we should. Ignoring the Holy Spirits prompting for, as an example to pray for someone, or to not offer help to someone when we can do so.
Failing to check on relatives or call a friend when the inspiration comes to do so.
Yes, it can be a grave sin or venial sin. Another example would be if you willfully neglected to say your daily prayers for one day (venial) compared to neglecting to say your daily prayers for a long time. Another one would be if you deliberately neglected to care for your family or the duties applicable to your state in life. Or deliberately failing to fast on days on which we should as by the precepts of the Church . Failure to give to the poor when you could do so is another example.
“I really ought to mow the yard today,” but you don’t mow it, that your not mowing it is an imperfection (in your goal of perfection),
That would be a sin of omission - it is something you failed to do knowing you should do it assuming you didn’t bother to mow it at all until it was really unkempt and not just putting it off until the next day, that is.
An imperfection as I understand it is when you do do the deed, but it is not done as well, promptly or carefully as you could have done it. So using the yard mowing example, if you still did mow the lawn that day, but procrastinated simply because of laziness (and not including reasons of weather, health etc) - so it was put off needlessly. But after some time, still went and mowed the yard, or you did but also did it negligently (so didn’t do as good a job as you normally would, ) then that would be an imperfection. You have done the deed (mowed) therefore it wasn’t an omission, but it was done a little carelessly and not as promptly as it should have been done - therefore an imperfection. Not done as perfectly as one could.
This article explains
Imperfection, Positive much better than my attempt to do so!
Sins of Omission by Archbishop, Cardinal Henry Edward Manning
Catholic Encyclopedia - Omission
Baltimore Catechism On Sin and Its Kind Q.276
"
What is our sin called when we neglect things commanded?
A. When we neglect things commanded our sin is called a sin of omission. Such sins as wilfully neglecting to hear Mass on Sundays, or neglecting to go to Confession at least once a year, are sins of omission."