Titles carry the post
On Reddit, most people decide from the title alone, and the title is the one part you can't change later. 300 characters is plenty; the risk is using too much of it. A title that states the question or the finding plainly is easier to judge at a glance than one that hides it.
Many subreddits add their own title rules: a tag in brackets at the start, a minimum length, or a format for questions. Those come on top of the 300 limit.
Long posts and comments
The 40,000 character body limit is rarely the problem; readability is. Break long posts into sections with headings or bold lead-ins, and put a short summary at the top (many communities expect a TL;DR).
Comments are capped at 10,000. If you are answering a question in depth, a comment near the limit plus a follow-up reply reads better than a wall of text.
Where SociaHive fits
SociaHive doesn't post to Reddit. It schedules posts to Instagram, TikTok, X, LinkedIn and other networks, and handles Instagram DMs automatically, for the channels you run as a brand.