Is your feature request related to a problem? Please describe.
avoidpartialspends and avoid_reuse are two important settings to avoid output linking, so I think being able to set them from the GUI would be a good improvement.
Describe the solution you'd like A GUI checkbox with a simple description on hover
The avoid_reuse flag can be set at wallet creation, but that dialog is already rather complicated. The flag can also be toggled for existing wallets. We don't have a UI to change settings for the currently selected wallet, but we probably should.
Another approach could be to just have that as a checkbox in the send dialog. As an aside: it would be nice if the send dialog checked if it was about to reuse, and then showed a little warning text, something like: "This transaction reuses an address which may hurt your privacy. You can avoid this by clicking here, but it reduces your available balance." This seems a better UX than hiding part of the balance, which is scary.
Adding GUI support for avoid_reuse is probably sufficient, because enabling that implies -avoidpartialspends:
-avoidpartialspends
Group outputs by address, selecting all or none, instead of selecting on
a per-output basis. Privacy is improved as an address is only
used once (unless someone sends to it after spending from it),
but may result in slightly higher fees as suboptimal coin
selection may result due to the added limitation (default: 0
(always enabled for wallets with "avoid_reuse" enabled))