as far as i can tell, camera quality in low end phones is really more or less the same in most of the cases. there might be a difference if one has multiple lenses, say one for zoom, one not. (a photo that is taken with optical zoom will generally have better detail than one that is taken with “digital zoom”.) there is also a little bit of a difference when it comes to the software / processing side of things. some phones have better stock phone apps than others, and more importantly, some phones do better HDR and other processing to provide better looking photos. the software processing that makes photos look better in some cases can make photos look worse in some cases though. so just make sure you know how to turn things like HDR off when, say, taking photos of things with really fine details like spider webs and such (or maybe shoot RAW instead and process yourself if that’s an option in those cases). more expensive phones will have things like TOF sensors that will help with photos in challenging situations and may have interesting future applications, but i don’t think those have made it to lower-end phones yet.
a consideration with photos is going to be how you’re going to manage them all. some phones will help to streamline the process of uploading them to a cloud share. (so, for example, if you’re tied to the Apple ecosystem, an Apple phone will probably do this the best.) others will provide things like removable SD cards that you could use to easily transfer files or increase storage, which can be nice if you plan to, say, take a lot of video or a lot of really high resolution or RAW photos. it may also be nice to see what kind of options (ex. USB C ports?) you have to physically connect your phone to your computer or, say, an external microphone or other devices. the downside of things like removable SD cards and USB ports is that they can reduce the water resistance of your device, since they provide places for water to enter. so tradeoffs.
maybe 5 years ago, some phones did struggle with locations, but i would be surprised if any recent phone – even a really cheap one – would struggle trying obtain a location. a phone with more processing power is likely to be able to handle processing of GPS locations a little more gracefully without heating up a lot and draining your battery though. so a phone with better processor specs or better battery is always going to be nice, i think, especially if you’re going to be heavy with your GPS usage.
i’m thinking that the processing power on even low-end phones – as long as they’re recent – should be enough to handle iNaturalist at the end of the day, though of course a more powerful one will get things done faster. i think the more important considerations that will make your experience a better one are things like a better screen (especially one, say, that you can see outside in bright sunlight). for me, an important consideration is the availability of updates. a lot of lower-end phones will be really slow with OS and security updates, if they provide them at all. so if that is important to you, then you should check on that before you buy. look for things like the durability of the screen, water resistance, and overall weather resistance, if you need that sort of thing, or else look for the availability of things like screen protectors and cases for the model you’re considering. finally, just make sure the phone is a good size for you.
the phone i have currently is a Pixel 3a, which i purchased new for US$299 during the holiday shopping season last year. at the time, the reviews seemed to indicate it had one of the best phone cameras available. (my opinion is that it’s fine and very easy to use, though my camera is much better for things like macro, long zoom, and fine details.) it handles GPS locations just fine, has a pretty good screen, and battery lasts me 3+ days for my use (very light, and i’m sure would be fine for a day’s worth of heavier use). because it’s Google-made, it’s updated frequently. in the US, one nice thing about the Pixel 3a is that it’s one of the less expensive phone that includes an eSIM, which in terms of cost means that i’m able to take advantage of the cheapest pre-paid voice/text and data plans that i’ve been able to find here ($0.09 per outgoing voice min or text, incoming is free, credit never expires, and additional $5 for 1GB data over 30 days). the only downside of the phone for me is that it’s only 64GB, all internal storage, though honestly, i haven’t used it enough to fill up the phone storage yet. it’s easy enough to connect the USB C to my computer to copy everything to an external drive though.
if i had not gone with Pixel, i would have probably gone with Moto G or Samsung A. Huawei and Xiaomi and even OnePlus aren’t necessarily viable options here in the US, unfortunately.
UPDATE: the Pixel 4a was officially announced the other day and is now available for pre-order at $349 USD. should be equal to or improve upon the Pixel 3a in just about every way (including price, since 3a is selling for $399).