If you truly, 100%, no conditions trust users who access your site, you can increase the default trust level from 0 to 1. That should help eliminate some of the roadblocks like limited links, uploads, spam checks, etc. Note that they’ll still be limited in the total number of topics and posts they can create in the first 24 hours, but not starting at TL0 will help.
Important Note for others reading this later: Unless you 100% trust your users (like a paid site via SSO), don’t change the default trust level - you’ll be opening yourself up to tons of spam.