I would invest in a basic lighting kit + backdrops and some reflectors. Since you’re planning to do portraits these will be very useful for you and you’ll need to get used to using them anyway if you plan on doing this for a long time. It’s also a relatively smaller investment than buying new lenses, etc.

I was a hobbyist as well and the biggest learning curve for me when I started freelancing was learning how to use my own lights correctly/creatively.

I’d also suggest renting out gear now and then and using it on your own time to get a feel for more “pro” stuff so you get a realistic sense of what you actually need.

This is fine as long as your pay factors that in the exclusivity, so have that conversation. I wouldn’t worry about it unless you really plan on using the photos yourself for whatever reason. Sometimes there’s just confidential stuff thats not a super big deal but they’d rather have control over.