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You're off to a great start with natural lighting. I don't think spending €150-200 on a large aperture lens will do much when there is no background to blur out.
I've not done much product photography but know that lighting can also be expensive. You could try using reflectors to create some fill light and lift the shadows. You can experiment with this on a small budget, e.g. this is £9.99 + postage. https://www.wexphotovideo.com/wexpro-30cm-collapsible-reversible-reflector-silverwhite-1584765/
If the natural light isn't strong enough to really reflect.. you can think about adding light, try a DIY softbox. https://www.diyphotography.net/how-to-make-a-diy-softbox-for-almost-0/
I’m not, perhaps that’s a more sensible place to start.
I don’t mind the pictures I’m taking currently, just think it’d look better when they’d be uniform in terms of white balance mainly. But that’s probably more a lighting thing than a lens thing now you mention it.