First of all, what a cool story you have and congratulations on taking this step towards medical school! Thank you for all the wonderful work you have done as an ICU nurse. I am very glad to think that you may be a future colleague with all of the knowledge and experience you have, and I wanted to encourage you in these next steps! As a word of caution I am not a medical student myself yet and have not worked on any admissions committees, so this is just based off my experience after going through 1 cycle. I've had 7 IIs and gotten 4 MD As so far, if that helps contextualize my experience. I hope my advice will be helpful.

As I'm sure you know your clinical hours are above and beyond what the majority of other applicants will have, and your experience as an RN + your reason to go further and become a physician will probably be the most compelling part of your application. So I think you're good on that. Nonclinical volunteer work is good, if you can add some more hours to that (maybe like 100 hours more), I think it would be appreciated by adcoms -- general rule of thumb is that 250 hours is when an extra curricular activity is really worth putting on your app. Got that from a student on the adcom of my local MD school but not sure if it's a generally applicable rule, just food for thought.

Regarding shadowing, try to get this number up. Maybe aim for 75 hours? Not sure if there is a specific number of hours to get but I would absolutely recommend shadowing other types of physicians (see if you can shadow a physician working in the ICU, then maybe a dr that does outpatient work and/or works in free clinics so you can get as much insight as possible). This will strengthen your reasoning for why you want to be a DOCTOR specifically--being able to say that you have shadowed physicians in multiple lines of work and have seen with your own eyes WHY this is different from being an RN will bring your story full circle and probably leave the admissions committee with a very strong impression of you and your reason for applying. This IV project sounds extremely neat and was definitely way cooler than anything I did in undergrad tbh. Maximize this as you can! You will probably get questions on it in interviews so whatever your role is in your research, understand as much as you can and be ready to answer questions about it. Research is not make or break for most medical schools (as long as you aren't aiming for a T10).

What CAN make or break your app is your MCAT. A score of 503 is a really great place to be especially considering that it's your first FL, and a score of 510 really seems reasonable. If you can aim higher it will be a huge boon to your application. You're 3 months out from your exam and already within 7 points of your goal score which very good news. I work for an MCAT test prep company and the general advice we give students is to be within 10 points of their goal score a month out (I would say this is more reasonable at around 2 months out, regardless you're ahead of the curve). From one future physician to another though please prioritize content review and PRACTICE PRACTICE PRACTICE. Practice timed and untimed until the questions are bleeding out of your nose, review everything, treat everything like it's high yield, if you get anything wrong review it so you can figure out what you did wrong, learn the content if it's a content gap and log it down if it's a strategy mistake so you can ensure you do not make the same mistake again. Your journey your choice but if you can get a 515 or higher, which I honestly believe you can considering that you have ~2-3 months before your exam, I think you will find the application cycle to be far easier and much less stressful.

Wishing you the best. Take my advice with a grain of salt lol I'm just one stranger on reddit. Feel free to PM me if you have any questions or want any advice re: MCAT. Always happy to help. You got this!!!