How to manually create options that can be entered as a sequence or list?

Question

I am trying to implement custom options in my code manually, rather than using the built-in Options tools. So I have functions that look like f[x_,y_,opts___] where opts is intended to be a (possibly empty) list or sequence of rules.
I'd like to be able to enter opts as a sequence or list, but I'm having a very hard time getting my code to cover both cases. How can I program my functions so that opts is converted to a list of rules, regardless of whether they are fed into the function as a list or sequence?

kglr · Answer

OptionsPattern[] does allow entering opts as a sequence or list:

Examples:
ClearAll[f]
f[x_, opts : OptionsPattern[]] := Plot[Sin[x t], {t, 0, Pi}, opts]

Row[{f[5, PlotStyle -> Red, Frame -> True, ImageSize -> 300], 
  f[5, {PlotStyle -> Red, Frame -> True, ImageSize -> 300}], 
  f[5, PlotStyle -> Red, {Frame -> True}, {{{ImageSize -> 300}}}]}, 
 Spacer[10]]

ClearAll[f2]
f2[x_, opts : OptionsPattern[]] := Framed @ Style[x, opts]

Row[{f2["ABC",
     FontColor -> Red, FontSize -> 62, FontFamily -> "French Script MT"], 
 f2["ABC", 
    {FontColor -> Red, FontSize -> 62, FontFamily -> "French Script MT"}], 
 f2["ABC", 
    FontColor -> Red, {{{FontSize -> 62, {{{{ FontFamily -> "French Script MT"}}}}}}}]},
 Spacer[10]]

How to manually create options that can be entered as a sequence or list?

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