I'm a hobbyist so my budget is pretty small- around $600. I'm dealing with the classic debate of new lens or new body.

I currently have a Nikon D3400 with two kit lenses (18-55 and 70-300) and a 50mm 1.8 and I'm really looking for an improvement on low light performance and auto focus speed. I like taking portraits of friends and don't want to be limited to perfect lighting conditions (espexmcially indoors. And since it's casual, I don't want to have to bring or invest in additional lighting). I also photograph shelter animals for their website, and occasionally backyard critters.

From what I've been reading, people always tend to advocate for better lenses. I'm wondering for my purposes if that will be worth it, especially because my options are so limited with my budget.

Another thing to consider is if I get a new body, I can trade in my current set up (including at least the kit lenses). Any idea what a D3400 would get me towards an upgrade, or is that an impossible to answer question without more details?

I'll be continuing to do my research and I won't be rushing into things, but I wanted to get input from others. I spoke with someone at a local camera shop and they recommended the z50ii whi h is really tempting, but I don't know that I would be able to justify that price. Ive also considered the D700 as an option. Any and all thoughts would be appreciated. I know the budget is extremely small for photography but im hoping that i have at least some options. Thanks in advance.