There are plenty of boost converters specifically for driving LED strings - the only real difference is the feedback voltage is lower so one uses a current sense resistor effectively making it a constant current boost. (normally 1.2-1.25V, but often CC boosts have a 0.3-0.6V feedback threshold).
Example:
https://www.mouser.com/ds/2/115/ZXLD1615-93645.pdfHowever, the reason I suggested a charge pump was the lower EMI. Even with shielded inductors, careful layout, switchers still tend to radiate more interference than charge pumps. A simple (high frequency) boost converter would probably be the most efficient, but you will have to test it with your metal detector - just because it has a high switching frequency doesn't mean it won't interfere. In fact you could probably just buy a cheap boost "module" from amazon/ebay set it to your desired voltage, and see if it interferes. If it doesn't, then you could probably get away with almost any boost converter. If it does, then you have to be more picky.
You don't have to have a constant current converter. Whilst efficiency will take a hit, using a 12V supply and a series resistor won't be that inefficient, especially considering you're going to run the LED's at 2mA - often switchers will have a quiescent current of 2mA+ so it won't get to 80%+ anyway.
So I say, if you can source some cheaper modules to test they might be everything you need, or at least highlight any issues.
https://www.ebay.co.uk/itm/5V-USB-Input-to-12V-Output-DC-DC-Step-Up-Boost-Power-Supply-Converter-Module-G/132613091764?hash=item1ee05b31b4:m:mUxjpGHBQMGT4hfCCCUQolQ(note, I have never used these, they could be noisy trash for all I know, just an example!).