Given that the budget is limited and you are not exactly sure about what kind of macro shooting you'll do, I think a wise solution would be to start with much cheaper options (compared to a dedicated macro lens).
E.g., you could get a MF 50mm lens (any brand/mount - doesn't matter, as long as it has a manual aperture control and undamaged filter thread), a cheap (~2-10$) reverse macro ring / coupler for the filter threads of your 55-250mm and the MF lens, use the coupler to put the MF lens in reverse position on top of 55-250mm, and voila - you get a macro lens with up to 250mm/50mm=5:1 magnification, and AF control. With this, you can do focus stacking experiments, like I did here:
For live (outdoor) macro, you can do a decent job by getting extension tubes with AF (ebay; 70$ all-metal, 50$ plastic), and using it with your 55-250mm. You get full lens control (AF, aperture), and significant magnification. (I get 3.7 more magnification, or 0.7:1, with my 135mm lens and extension tubes.)
You don't list a flash in your gear - you must get a good one if you plan to do macro. I think your macro money will be better spent on a flash, plus some simple diffuser (I did it myself from a plastic container from a dollar store.)