You are not "overthinking," though perhaps you are in need of encouragement to keep writing. Many others have had these self-same questions. In my research and reading to learn "how to write" I came across the idea of writing a "character Bible."
Character Bible and Wardrobe Charts
A character Bible is a set of files in which one writes up all the details about each character such as gender, age, height, interests, place in family, and all the important milestones in that person's life. This may include items that don't enter the story but help make the character who they are. For characters in serials, this is especially important so that the author can go check if Tom had glasses in Book 2 or if that only happened in Book 4. Or maybe it was Harry who had the glasses and Tom who wore a baseball hat everywhere he went, even to church if his wife--or was it his mother--let him.
You mention "forgetting details." This means you had them in your head and/or imagination at one point. Write them down right away. No need to draw; write them down in all their tedious detail. Make a list or chart if that is helpful. People divide their wardrobes into headgear, tops (e.g. shirts, blouses, jackets), bottoms (e.g. slacks, pants, jeans), footwear (different kinds of footwear for different purposes), accessories (e.g. ties, scarves).
I am not into clothes for my characters--they just wear the same drab stuff all the time with a focus on what's going on, but some authors dress their characters in different clothes every day. If you want to do that, I can visualize a chart for each character. Across the top, write the different categories of the wardrobe and down the side write the names of items, leaving room to list a variety of jeans, shorts, slacks, etc. Then, when it comes time to dress Tom for the government meeting or Therese for the party, all you have to do is go into the "closet" aka chart and pick from what's there.
I use pictures, too, for clothing. Since I'm writing about people in earlier decades, I'll ask Google for "girl's dress 1960s" or "men's clothes 1940s." That tends to bring up Sears catalogue pages from the years requested. For the centuries before photographs, it tends to be paintings. I have not gone back far enough to need pottery or cave engravings but I think that's where information of the very ancient styles come from. To not infringe on copyright of contemporary photographs, one can pick and choose elements of the garments to describe. You are not reproducing the photograph.
How Much Description
This brings us to another of your questions: How much description is required?
I personally don't like reading long detailed descriptions of clothing when I'm dying to know who killed the corpse we met in Chapter 1. A friend suggested to use just enough detail to get the reader thinking in the right direction. Let readers fill it in with their own imagination.
For example, Tom with the baseball cap in church was probably wearing some kind of pants and footwear though we are never told. I'm making this up as I go. All we know is that he was in a t-shirt with a tie and baseball cap when his mother caught him going out the door. Given that description of his dress from the waist up (I'd add colour and design in a real story), reader imagination will dress him from the waist down. We want to know what happens when this guy gets to church, especially if it's a traditional suit-and-tie congregation.
Describing A Large Group
Don't try to describe every person in a crowd of a hundred people. Describing the six key men in your group is enough. Authors use various techniques to describe groups. Often they start with a characteristic everyone had in common, then add a bit more to give the reader a general idea. Include enough detail to carry the plot. Maybe all six men carried a briefcase or wore a tie. Maybe in your large crowd everyone is wearing a uniform or religious symbol or is "prepared to take a stand." You can use clothing and body postures to set the atmosphere of the scene (calm, tense, angry, joyful, etc.). For example, a scene with a priest who boldly displays his cross and a lawyer who pushes out the chest of his expensive suit as he struts up to him with a briefcase both describes people and suggests conflict.
How I Get The Details Down
You ask how I get the details down. Often the details are the last thing I add to the scene. I'll visualize in my head the exact location (room, lawn, etc.) where the characters are, what else is in their immediate environment, what tools they are using, what clothing they are wearing. Then I'll add enough to make the scene come alive as described above.