A couple years ago, I bought a cheap used DSLR off of eBay. It was a good starter camera, but I quickly noticed some problems with the meter/sensor. Whenever I tried to use any automatic exposure modes (eg aperture/shutter priority or even auto ISO) the metering was pretty much random, with some images almost pure white and others pure black.
This was good in a way because it forced me to learn manual exposure early on, instead of relying on the camera to calculate it for me. However it was also a pain in the ass when taking photos on hikes without a flash. Going between sun and shade, I'd have to waste a few shots on exposure calibration. Since DSLRs don't have exposure previewing, it was a time-intensive process. My fast-moving subjects often left the scene before I could get a decent shot. (This wasn't all the camera's fault; I'm a beginner and I'm sure a more experienced person could have made it work!)
I was still happy to persevere through these issues, until some of my shots started coming out with obviously distorted colors -- many were extremely green, and while the RAWs could be massaged into a better color-scheme, they were missing depth in the blue and red channels. It wasn't every shot, but maybe 5%, which is terrible when that 5% includes your favorite shot of the outing. I'm pretty sure this was a sensor/computer issue that effectively totaled the camera.
At this point, graduation was coming up (May 2024), and I got some financial gifts from relatives! Basking in my extreme luck and privilege, I decided to get rid of the money fast by purchasing a new camera setup.
My new camera is a Canon EOS R50, which is a mid-range mirrorless that cost me about $500. I also purchased the RF100mm F2.8 L Macro lens, which was normally $1200, but I found an eBay deal for just under $1000. (Plus an RF-to-EF adapter for some EF-mount lenses.) In terms of lighting, I found a Canon Speedlite 580EXII for cheap-ish on eBay (which needed an adapter), and I spent around $150 on an AK Diffuser. In all, this cost about $2000.
Overall, I'm extremely happy with how everything has performed. The difference in photo quality was noticeable pretty much immediately after my new gear came in. The RF100mm F2.8 L Macro is one of the best macro lenses on the market, with a maximum magnification of 1.4x while most macro lenses top out at 1.0x. It has delivered absolutely stellar photo quality, with virtually no noticeable distortion or chromatic aberration at any distance.
I don't mind the R50's crop sensor -- it may actually be a plus for macrophotography, since the pixel density is higher than on a full-frame camera. However, the R50 has some weird design choices when compared to Canon's higher-tier product lines. For example, when the camera is turned off, the R50's shutter stays open, meaning that the sensor gets dusty while changing lenses.
The biggest bang-for-buck increase in photo quality came from the lighting, though. It's hard to overstate how much difference the diffuser made. I'd been trying out homemade box diffusers, and I was already seeing good results, but the professionally-made ones really "wrap the light around the lens," leaving you with beautiful even lighting. On a casual hike, I'll leave the flash setup at home since it's hilariously bulky, and I can get decent photos with quite a bit of detail and color. But nothing really compares to photos taken with a diffused flash.
If I could go back and spend my money differently, I might go with an Olympus setup instead. They're lighter-weight since the sensors are so small, which means that it could be practical to hold the camera in one hand while stabilizing a twig/leaf/whatever with the other hand. They also have decent sensor density, and there's a solid ecosystem of compatible macro lenses, often cheaper than RF-mount ones, including some crazy high-magnification macro lenses.
Overall I'm very happy with my current camera. At this point, the biggest factor holding me back from even better photos is my skill level, not my camera, which is a great place to be! It means that I can focus on being out in the field and taking photos rather than troubleshooting my camera or wishlisting lenses on eBay. I have everything I need.