Toxic heavy metals is not a usual problem for backpackers in the USA. Sawyer is a pretty simple low-cost system so the manufacturer may not be interested in after-filters. You would have to contact them and ask why.

Improving the "taste" of water is pretty subjective. I never have had the need. The Sawyer squeeze will filter the organics which often give the water a bad taste. It will not change the pH or take out metals. Snowmelt water can taste funny and flat.

I liked my Sawyer Squeeze but the washer on the screw cap fell out and went missing. It was only $25 and I got two years use from it. If I buy another I will try the mini.