For the record, I have nothing against buying a Canon or Nikon for this purpose, but I think it might be worth putting this in perspective. There is very little that is "special" about Macro photography in the size range that you have stated. Except for the resolution limit of your camera (about 10MP) you could do as well simply by buying a Macro lens.
I am uploading a couple of images I took this afternoon on the spur of the moment using my Yi-M1 with its 42.5mm short tele/Macro lens, which, if you can find one (they are sold out at
B&H, where I bought mine) costs about $400 US including the lens.
[added 2017-11-27]
I forgot to mention that the Yi-M1 I used for the above photos is a "Micro 4:3" camera with a smaller sensor than your A300. You have an APS-C sensor. The equivalent lens (to give the same angle of view) would be about 50-55mm.
See below for some added comments.
As cheap as this setup is, I can assure you that no camera with a sensor up to around 24MP is going to do better. Other photographers might do a better job, and produce a better picture, but most DSLRs and Mirror-less cameras should be able to handle the task.
Aside from what you are considering, you might consider buying another Sony, so you can keep the old camera and share lenses. If you buy a Sony mirror-less, then you can buy an adapter and use any lenses that might have for the A300 on the Mirror-less too. The auto-focus version lens adapter is pricey though, so I would buy the adapter that only supports manual focus. Right now, there seem to be a lot of used lenses around for the A300 and those series cameras.
But again, there is nothing wrong with buying a Canon or Nikon. . . .
Images:
Most of the images were re-edited from the stock JPEG files with NO adjustments (except cropping or resizing).
I also included 1 "detail" crop from from the RAW file which has been modified. Actually, the edited JPEGs are better in this case because I did not spend that much time on the RAW.
"PB240006.JPG"
Partial EXIF
Width 5184
Height 3888
Bit depth 24
Color Representation [AdobeRGB]
F-stop f/7.1
Exposure 1/320 sec.
ISO: 200
Exposure bias: 0 step
Focal length 43mm (42.5)
Max aperture 3.65
Metering mode "Center Weighted Average"
35mm focal length 86
Size 6,337,767 bytes
"PB240006-jpg-rs1640-C1.jpg"
Resized from camera generated JPEG image. "Color representation" has been converted from "AdobeRGB" to "sRGB" by Corel Paintshop Pro, but no other alternations were made to the image.
"PB240006-jpg-Crop01-C1.jpg"
A full resolution detail crop from the original JPEG file of the above image.
"PB240006 -1b-DetailCrop01-C1.jpg"
A full resolution detail crop from the DNG file for the same image. This file has been altered to match the colours and appearance of the JPEG files.
"PB240016.JPG"
Partial EXIF
Width 5184
Height 3888
Bit depth 24
Color Representation [AdobeRGB]
F-stop f/7.1
Exposure 1/400 sec.
ISO: 200
Exposure bias: 0 step
Focal length 43mm (42.5)
Max aperture 3.65
Metering mode "Center Weighted Average"
35mm focal length 86
Size 6,607,326 bytes
"PB240016-jpg-rs1640-C1.jpg"
Same subject but different lighting and background.
"PB240016-jpg-Crop01-C1.jpg"
Detail crop of the above image.