First before buying any “starter kit,” buy a copy of “Closeups in Nature” by John Shaw and read it cover to cover. Written in 1987, it’s a film era book but extremely well written and clearly and simply explains all the pros and cons of various options - extension tubes, closeup filters, macro lenses, lens reversals, plus lots of field techniques. Obviously doesn’t cover modern digital techniques like focus stacking but it is a very clearly written and comprehensive book on optical techniques.
https://www.amazon.com.au/Close-Ups-.../dp/0817440526
I note that he also wrote a book on digital nature photography in 2017, "John Shaw's Guide to Digital Nature Photography," which I don't have and have not read but I assume he covers many of the digital advances in that book in a similarly clear manner.
If you decide to buy a macro lens for your DSLR:-
Tamron 90mm f2.8 macro.
Available in all mount types and well regarded by many nature photographers and won’t break the bank. See photos taken with this lens attached.
Joe