Craft macro to centralize the markup and config for responsive images

img.html

{# ================================================================== #}
{# Responsive Image Macro
{# ================================================================== #}
{# 
   Macro to centralize the markup and config for responsive images. 
   
   Based on an article by Marion Newlevant (@marionnewlevant) and 
   adapted for the Lazysizes plugin. Read more: 
   https://straightupcraft.com/articles/responsive-images-with-twig-macros
   
   Relies on the following Craft plugins:
   Imager: https://github.com/aelvan/Imager-Craft
   FocusPoint: https://github.com/smcyr/Craft-FocusPoint

   Relies on the following front-end plugin:
   LazySizes: https://github.com/aFarkas/lazysizes
#}

{# ================================================================== #}
{# Srcset
{# ================================================================== #}
{# 

  {% set options = {
    alt: alt | default(''),
    class: class | default(''),
    img: img,
    imgType: imgType | default(null)
  } %}

  {{ macroImg.srcset(options) }}

  Available options: 
  @param alt (string)                 - [optional] Text for alt tag
  @param class (string)               - [optional] Classes to apply to img
  @param dataExpand (string)          - [optional] Sets custom Lazysizes data-expand attribute
  @param effectType (string)          - [optional] Determines if Imager should add effects to the transforms. String should match case in switch statement below. See Imager docs for more info on what's possible. 
  @param img (asset)                  - [required] Craft image asset
  @param imgType (string)             - [required] Determines which transform settings should be used to generate srcset markup. String should match case in switch statement below.

{# ------------------------------------------------------------------ #}

{% macro srcset(options) %}
  
   {# Set some defaults #}
   {% set defaults = {
      alt: null,
      class: '',
      dataExpand: '400-800',    
      effectType: null,
      imgType: null,
      sizes: 'auto'
   } %}

   {# Merge options with defaults #}
   {% set options = defaults|merge(options) %}

   {# If necessary, define transform effects #}
   {% switch options.effectType %}
      {% case 'greyscale' %}
         {% set effectsArray = { grayscale: true, contrast: 0 } %}
      {% default %}
         {% set effectsArray = { } %}
   {% endswitch %}

   {# Define transform settings based on image type #}
   {% switch options.imgType %}
      {% case '1x1' %}
         {% set imageRatioWidth = 1 %}
         {% set imageRatioHeight = 1 %}
         {% set mode = 'crop' %}
         {% set imageSizes = [ 
           { width: 1200 }, 
           { width: 900 }, 
           { width: 600 },
           { width: 300 }
         ] %}

      {% case 'carousel' %}
         {% set imageRatioWidth = 2 %}
         {% set imageRatioHeight = 1.25 %}
         {% set mode = 'crop' %}
         {% set imageSizes = [ 
           { width: 2000, ratio: 2/1.25 },
           { width: 1600, ratio: 2/1.25 },
           { width: 1000, ratio: 1/1.25 },
           { width: 600, ratio: 4/3 }
         ] %}

      {% default %}
         {% set imageRatioWidth = 1 %}
         {% set imageRatioHeight = 0.76 %}
         {% set mode = 'fit' %}
         {% set imageSizes = [ 
           { width: 1600 }, 
           { width: 1200 }, 
           { width: 800 }, 
           { width: 500 }
         ] %}
   {% endswitch %}

   {# Set Srcset Transform #}
   {% set transformedImages = craft.imager.transformImage(options.img, imageSizes, { mode: mode, effects: effectsArray, position: (options.img.focusPctX ?? '50') ~ '% ' ~ (options.img.focusPctY ?? '50') ~ '%', jpegQuality: 80, interlace: true }) %}

   {# Output lazysizes srcset markup #}
   <img class="{{ options.class }} lazysizes" 
        alt="{{ options.alt ?? options.img.title }}" 
        src="{{ craft.imager.base64Pixel(imageRatioWidth, imageRatioHeight) }}" 
        data-sizes="{{ options.sizes }}" 
        data-srcset="{{ craft.imager.srcset(transformedImages) }}" 
        data-expand="{{ options.dataExpand }}"
      >

{% endmacro %}

Epic! This is exactly what I needed. Thank you @brettburwell 🙇