The WMF needs to know how well the editing experience is working, both for VisualEditor and the wikitext editors. We should be asking editors for their feedback all the time, not just if the user is signed up for research.
I request a simple, single-question microsurvey. It should take the editor no more than a few seconds to either answer it or cancel it. If
My preferred "trigger" is upon saving an edit. A generic question like "Would you recommend editing Wikipedia to other people?" might be best, with a simple Likert scale  or perhaps yes-maybe-no options.
I want the data automatically collected and anonymized. Ideally, it would record some basic information about the account, such as the editing environment just used, whether the editor is logged in or logged out, the approximate age of the account, approximate number of edits, and/or userrights (e.g., autoconfirmed), so that it's possible to see whether brand-new editors have different views compared to experienced editors.
This should not require a major effort in terms of engineering, since there are some existing tools that could be adapted to collecting the information.
If the rate is set to just 1% of edits (or even lower), with a cookie (or account setting, for logged-in users) to prevent someone from being asked more than once or twice a month, then we should be able to get enough information without spamming people all the time. A preference to opt-out might be ideal; alternatively, it could be possible for logged-in users to hide it via personal CSS or JS code.