Awesome! Congrats on the lens & tubes. I have a D750 I use for landscape/portraits but for macro, I’ve got an Olympus OMD EM1 with their 60mm (120mm in 35mm equivilance) & extension tubes.
(I have to apologize, I’m having a hard time putting down good recommendations -hope this helps a bit, though)
One of the big challenges in macro is depth of field & yes, I think you’ll have a challenge with lighting, too but nothing you can’t work around! And your pop up flash may help more than you’d think... I would recommend setting up on the tripod & working with the lens first before adding the extension tubes. With the tubes, you’ll have to get VERY close to focus which will probably render the flash ineffective. I’d probaly start shooting at f/5.6 -it will allow a good amount of light in & the depth of field will be reasonable on your 7500. You should get plenty of light at that aperture. Work your way up from there -if your on the tripod, you won’t have to worry too much about camera shake. Speaking of which, set your 2-second delay when your on the tripod, too.
I bet you can get some good shots handheld with the pop up flash.
Try manual focusing when in the tripod, especially with the extension tubes.
Higher ISO settings may help.
My girlfriend is coming over because she’s interested in shooting close up -I’m sure I’ll have better organized tips after we’re done & I’ll post them here a bit later.