i do pretty much all my studying in big chunks during breaks, so i say go for it. 10 grammar points daily is definitely doable. I did n5 and n4 before my free trial even ended. I started like in late june and had school in august so i didnt do much beyond that. After that during winter break i did 10 a day for n3 until i finished. my goal for this summer is to finish n2 and some of n1 (im only doing like 6 a day tho). make sure ghosts are set to minimal if you plan to do this (i think its the default setting)
I would recommend doing one big session in the morning right when you wake up (this is when you add your cards) and one at night before you go to bed (just to kill ghosts and move your cards along). this will take some time (a couple hours maybe) once the reviews start piling up. just know what your signing up for
also, i would say taking a hit on retention is worth it just for more grammar points. I would much rather know 600 grammar points at 70% retention than only know 300 grammar points at 80% retention. you just know so much more its worth getting a few more answers wrong
doing 50 words daily not something ive done before (i do 25 new cards a day in anki). Its doable, but make sure you don’t spend too much time on each card. if you are spending 10 seconds per card while adding 50 cards a day you will want to die. its okay to get a few more wrong just keep answering quick is the most important part. if you ever feel burnt out, dont skip reviews entirely, just stop adding new cards until they get manageable. I use anki and theres a speed focus add on to make sure you answer fast, but idk if jpdb.io has that
My adding 25ish cards a day takes maybe an hour (but it could be faster), so i would estimate it would take maybe 2 hours of reviews daily, give or take time based on pace. youre probably gonna have to slow down on grammar a little bit, but i could see you finishing n2 even with some slowdown. just keep in mind that this will be like a part time job, my estimates have maybe 4-6 hours daily just on reviews for you at this pace, so if you can do that on top of whatever time you spend reading go for it.
my estimates might be off, but if you ever start getting burned out just stop adding new grammar/vocab for a week or so and then you should be good. maybe you can restart a bit sooner on the vocab, but i would wait a week to kill off a lot of the ghosts for bunpro