I have the same issues. What I can tell you first is stay away from the stimulant laxatives (Bisacodyl, Senna, etc.)as these are very bad for your colon. These should be your VERY LAST option.
Stool softeners are more safe (Miralax, magnesium citrate (liquid), etc.).
Fiber (or lack of) is very rarely the cause for constipation in my opinion. It MAY help but in my 5 years with the issue its never helped to much, and some fiber makes it worse.
What has been helping me recently is electrolyte power and Inulin (Miracle Fiber). I’ve only been using this for a few months and im always skeptical.