Congrats on finding the old Minolta for macro. If your normal lens
doesn’t satisfy you, a cheap enlarging lens adapted to a bellows or
your extension tubes should work great. You might try reversing the
normal lens first, there are reversing rings available for most
camera brands. If that doesn’t suit you, there should be some way to
mount an enlarging lens to the tubes. At near 1:1 ratios, it might
work even better reversed (a 50 mm lens), so think reversing ring or
something glued up for the purpose.