You can get a 1nm resolution "science grade" spectrometer for under £1k - I used to design them [1]. You can also buy used ocean optics kit for very reasonable prices on eBay (a few hundred for a USB2000 if you wait).

Or make your own? It'll probably be good enough https://publiclab.org/w/spectrometry

ThorLabs also sell cheaper spectrometers by the way. You don't need a spectrum analyser with seven significant figures! https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=34...

You can also get light testing devices used for film/industrial illumination https://www.sekonic.com/color-meter/c700 (about 1.5-2k)

Or even single chip spectrometers with broad 20nm resolution from Hamamatsu (about £400 on group gets). https://groupgets.com/manufacturers/hamamatsu-photonics/prod...

[1] https://is-instruments.com/products/miniature-spectrometer/