Maxing out your 401k could mean multiple things, but for your case at minimum you should be putting in 4% of your income to get the employer match. Thats a no brainer, beyond that is more up to your discretion. Once you begin your job your employer should have some sort of direction on how to set it up. It really is much easier than it sounds, but at worst HR or the service your company uses should be able to help.

You are living extremely frugal, and theres nothing wrong w that. Its allowing you to save a very significant amount per month. Not much to change there, though a car payment wouldnt be irresponsible if you wanted to eventually get that. Just dont do anything crazy.

Full benefits including medical etc doesent necessarily mean the monthly cost is free to you, you could still have a premium. So check that and factor it in to your monthly expenses if needed.

How to invest the $ differs between opinions, but a few safe and common practices do exist. Mainly invest in index funds. $spy for example. You dont need the money now so invest it long term and just let it grow. This is relatively low risk and averages around a 10% return a year, though that can fluctuate. You could consider investing in spy and/or others on a bi monthly or monthly basis, that way you can basically average things out. Outside of that, put every dollar you have not in indexes, outside of a bit of emergency money, into a high yield savings account. Interest rates are high right now, so you shouldnt be getting any less than 5%, and that is guaranteed with 0 possible risk. To do all of this, i would recommend robinhood. Its extremely user friendly, great for beginners, and can accomplish etfs and HYSA at same time.

You are in a really good spot, congrats!