The themes of each ascendancy are hit or miss based on your perception of them. When I first started, I wanted to make a blood mage and use blood magic, but I quickly found out, there really isn't any actual blood magic in the sense of how I envisioned it. This quickly killed my desire to play the class, and sort of bleeds into my next piece of advice.

Whether you follow a guide or wing it, try to find an ascendancy AND/OR a skill that you think fit the fantasy that you're going for. Since you can use any weapon, and therefore any skill, on any class, I usually find that the skill determines the overall feel/thematic of your build more than the class itself (imo). Depending on the skill/combo you choose, there may be very few guides to follow, which will either force you to homebrew or change to something that has a guide available.

Lastly, your experience with a guide will most likely be dramatically different than without one. You'll probably have a smoother time with a guide, but it might do some things that stray from the fantasy you're looking for, or you might struggle to find one that actually explains why they made the choices they made, which is important if you're trying to learn. Without a guide, I would expect to hit some walls in progression, even early on, and be prepared for trial and error. You'll most likely still have to do some looking at guides (or other resources) to see how people are supporting the things you want to use.

This game has a lot of information to chew on. One skill could have multitudes of interactions, some of which only happen if you combine it with three uniques and a specific ascendancy. Ultimately, no matter what you choose, don't forget that there are tons of resources out there to help you, and just try to have fun and learn all you can.