It may take you a few years to figure out how you want to rotate your stock to meet whatever goals you have.
I rotate my old birds out every fall (at 2.5 years old) and hatch new chicks every early spring so I have pullets to lay thru every winter with lights going.......
......that's the short version of my flock management.
Feather picking can be caused by several things....crowding from not enough space for number of birds, insufficient protein in feed(s), or in the end maybe just bully birds.
If you have bloody birds, you need to address the cause of the pecking asap, and separate the bloody birds if at all possible...
.......blukote might help, vaseline and DE is not likely going to solve the problem.
First year of chicken keeping can be overwhelming, like getting a sip of water out of firehose...lots to learn.
Just keep reading, evaluating advice given(ignore what doesn't help-acknowledge what does), and trying various things until you find what works in your situation.
Bet of cLuck to yas!
I, too, take the two laying cycle approach to my flock - with new pullets starting each spring so that I have eggs through the winter from the first year birds and have first and second year birds laying through each summer. You can sell hens at the end of their second laying cycle pretty easily around here and make back more than enough to pay for the replacement chicks in the spring. I don't even bother with lights as the first year birds that I have carry right on through the winter with no encouragement and the older birds take the winter off to come back gangbusters in the spring.
Definitely need to sort through to the "why" of the feather picking (as noted, there are several reasons it can become an issue) because addressing the cause is the best way to solve the problem, you can put bandaids on it in the meantime to try to mitigate the damage being done, but only eliminating the cause will provide long-term relief to you/your birds. In some situations if the habit has become firmly entrenched it is all but impossible to reverse - because at that point it isn't even about whatever caused it to start as it has now simply become a routine/habit for the birds to continue doing it. If the plucking is only directed at the backs you may want to try a hen saddle - a piece of cloth that goes over the back area to protect the feathering/skin underneath. It is intended to protect hens from the damage done by a rooster during the mating process, but would also help in a plucking situation where back feathers are the target (the culprit *may* target another body area, but it may be just the ticket to breaking the cycle). I understand your thought process on the vaseline idea, but the issue there is it will create a shiny area on the backs of the birds and this will actually attract more attention, rather than less.
Can you tell us a bit more about the coop you have? The fact that it came to you without much ventilation and you have nest box sleeping and feather picking issues have me wondering if perhaps you received a unit that is causing some of the issues you are dealing with (which you are working to change, and that is a testament to your determination to learn and improve things for your birds). There may be some additional easy changes you can make that would help you with the problems you are having.