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I am current using google maps API to plot points on a map. I am looking for the best way to achieve a 'lozenge' type effect between points (see image).
Essentially the thickness of the lozenge will be determined dynamically.

The only way I can see to do this, is to draw 2 circles at the ends of my points, and create a rectangle in between them, with no borders. I don't like this idea, as it essentially adds 3 polygons to my map for each line I have, and means that I would have to then add 3 click events for each line (one on each polygon) to display data that I want to show, when clicking on the line.

Is there any better way to achieve this?

Just create 2 Polylines...

function initialize() 

 var map = new google.maps.Map(document.getElementById('map-canvas'), 
 zoom: 3,
 center: lat: 20, lng: 0,
 mapTypeId: 'terrain'
 );

 var flightPlanCoordinates = [
 lat: 20, lng: -20,
 lat: 20, lng: 20
 ];
 
 new google.maps.Polyline(
 path: flightPlanCoordinates,
 geodesic: true,
 strokeColor: '#FF0000',
 strokeOpacity: .3,
 strokeWeight: 20,
 map: map
 );
 
 new google.maps.Polyline(
 path: flightPlanCoordinates,
 geodesic: true,
 strokeColor: '#000000',
 strokeOpacity: 1.0,
 strokeWeight: 1,
 map: map
 );


initialize();

#map-canvas 
 height: 200px;

<div id="map-canvas"></div>

<script src="https://maps.googleapis.com/maps/api/js"></script>

Edit:

You mentioned in your comment you need the stroke to be fixed at a given distance from the center line.

You could still use that technique I think, as you can calculate the size in meters of a map pixel, based on the latitude and the zoom level, and as the size of the stroke is also in pixels, you should be able to recalculate the stroke each time you zoom-in/out. It won't be 100% precise since the stroke width must be an integer, but it should work.

It should work if the various Polyline points are more or less on the same latitude, because the resolution of a map with the Mercator projection is dependent on the latitude. So, for example if you have a big Polyline crossing from -70 to +70 latitude, it won't be accurate all the way.

This is the formula you can use to compute the size of 1 map pixel. You would need to replace map.getCenter().lat() by your Polyline center point.

// Calculate the width of 1 map pixel in meters
let scale = 156543.03392 * Math.cos(map.getCenter().lat() * Math.PI / 180) / Math.pow(2, map.getZoom());

Unfortunately, there is an issue with this approach. Although it's undocumented, a Polyline stroke width can't be bigger than 32px.

var map;
var polyBounds;
var polyStroke;
var polyWidth = 50; // Width in meters

function initialize() 

 map = new google.maps.Map(document.getElementById('map-canvas'), 
 zoom: 12,
 center: 
 lat: 20,
 lng: 0
 ,
 mapTypeId: 'terrain'
 );

 var flightPlanCoordinates = [
 lat: 20,
 lng: -0.5
 ,
 
 lat: 20,
 lng: 0.5
 
 ];

 polyStroke = new google.maps.Polyline(
 path: flightPlanCoordinates,
 geodesic: true,
 strokeColor: '#FF0000',
 strokeOpacity: .3,
 strokeWeight: 0,
 map: map
 );

 new google.maps.Polyline(
 path: flightPlanCoordinates,
 geodesic: true,
 strokeColor: '#000000',
 strokeOpacity: 1.0,
 strokeWeight: 1,
 map: map
 );

 polyBounds = new google.maps.LatLngBounds();

 for (var i = 0; i < flightPlanCoordinates.length; i++) 

 polyBounds.extend(flightPlanCoordinates[i]);
 

 google.maps.event.addListenerOnce(map, 'idle', setStroke);
 google.maps.event.addListener(map, 'zoom_changed', setStroke);


function setStroke() 

 // Calculate the width of 1 map pixel in meters
 let scale = 156543.03392 * Math.cos(polyBounds.getCenter().lat() * Math.PI / 180) / Math.pow(2, map.getZoom());

 polyStroke.setOptions(
 strokeWeight: Math.ceil(polyWidth / scale)
 );


initialize();

#map-canvas 
 height: 200px;

<div id="map-canvas"></div>

<script src="https://maps.googleapis.com/maps/api/js"></script>

So unless you find out that these values allow for your use case, this won't be a viable solution...

The solution you were pointed to in the comments (How to draw a polygon around a polyline in JavaScript?) is still probably your best option.

I actually discovered a much easier way to achieve this using the google maps API.
There are built in functions for getting a bearing of a point based on a start point...
So I created a function that takes the start and end points of a line, and then creates points around these based on the heading - then create a polygon using those points.

var pointCount = 12;
function setLozengePath(startPoint, endPoint) 
 var sp = google.maps.geometry.spherical;
 var heading = sp.computeHeading(startPoint, endPoint);

 var points = ;

 for (var i = 0; i <= pointCount; ++i) 
 points.push(sp.computeOffset(endPoint, radius, heading + 90 - i * 15));
 

 for (var i = 0; i <= pointCount; ++i) 
 points.push(sp.computeOffset(startPoint, radius, heading - 90 - i * 15));
 

 return points;