I encourage you to work through or at least watch through the fundamentals courses to make shore up any missing skills or ideas.  Then pick a style course that most closely matches the style of music you want to play.  My guess is blues or rock since the Dead is the band you mention.

Regarding the specific skills you mention.

Playing better rhythm is something that happens with practice.  Pick a course, work through the lessons and practice regularly.

Playing fills in between chords is a specific topic I cover in these tutorials.  They start simple and gradually get more advanced.

https://www.guitartricks.com/tutorial/170/

https://www.guitartricks.com/tutorial/918/

https://www.guitartricks.com/tutorial/1147/

https://www.guitartricks.com/tutorial/2380/

https://www.guitartricks.com/tutorial/2768/

These tutorials cover the basics of improvisation:  figure out the key signature (or scale) to use, pay attention to the chord progression and target chord tones, build melodic phrases.  A lot of this comes down to building vocabulary.  Again they start wimple and gradually get more advanced.

https://www.guitartricks.com/collection/learning-to-improvise

Hope that helps!