I’m not a designer, but I do freelance, so at least I can lend a hand here. I agree with @Mofodie. charging pre project is very difficult to me, particularly when it involves so many pieces. At minimum, I’d go through each task, decide how much time it would take, then double that time for an estimate.
If they insist on paying you per project and not per hour, then just give them this number. I prefer to give them the rough estimate and then keep them updated as I work in case anything is taking me longer than I initially said. I try to work like my mechanic… give them the rough idea, and keep them updated, but don’t make any promises… you just don’t know until you start working.
To make things simple- you can just take your salary (or what you think is fair given your location/experience level/ etc) then divide that by 2 to get an hourly rate - so an $80k salary roughly equates to $40/hr.
Of course, a salaried job gives you lots of perks, like days off, discounts, software, internet, workspace, etc.
I’ve heard some people say- take that rate, in this case, $40/hr, then double it, to get what you actually should be charging. Bc the cost of doing freelance has a lot of costs unrelated to the actual work, like responding to an email related to the gig or doing your own invoicing. If you are billing 20 hours of freelance you might actually be doing 30 hours of work to maintain your computer, etc.
Buuuut, I would counter that with- what are you getting from this job (learning opportunities, connections, resume additions)? And what’s realistic to ask for? It’s not an easy time to try to ask for a lot of $$ from freelance clients, but I think the good clients will pay you fairly, probably not extravagantly.
I like to see that doubled hourly rate as my goal rate. I rarely get my goal rate, but, if it’s a client who is offering me very low $$, I respond with this high number, and usually we can meet somewhere in the middle. It works for me and them, so everyone is happy 
I have, on occasion, taken gigs which pay me a lot less, but offer me a chance to do something or work with someone I know will give me a great learning opportunity or push me into learning a new skill I probably wouldn’t really put the effort into learning on my own.
There’s no right answer here- but I hope this is helpful!
I would also check out Sew Heidi, she has a lot of freelancer videos and classes, if you need a confidence boost or just some basic guidelines on how to set up invoicing etc, she’s definitely a good place to go! Good luck!