In this COVID-19 situation I guess many photographers keep staying home and reprocessing old photos. I am reprocessing photos taken about 10 years ago. At that time my gears were SIGMA DP1 and SIGMA DP2.
Google Nik Collection is a free software. Nik Collection includes “HDR Efex Pro”, my favorite.
It works standalone. But its value will be increased if combined with Adobe Lightroom or Photoshop.