Sorry book fantasy not the other.

Any suggestions on fantasy books, trilogy or series preferred, magic related, but the main character for a change is an accomplished wizard, sorcerer already?
What books/series have you already read that you've enjoyed? This will give us an idea of what you like.

I recommend...

he Dresden Files series by Jim Butcher
Harry Potter series by J.K. Rowling
Spellsinger series by Alan Dean Foster (the Spellsinger in question is not accomplished but it's a really fun series)
Recluse series by L.E. Modesitt Jr (not all stories feaure magic but a good number do and it's a really good series)
Ironfoot, by Dave Duncan meets your requirements; https://www.amazon.com/dp/B07H9FQFKJ/

Also try The Blacksmith's Son by Michael G. Manning (which meets your requirements). Alvin the Maker might also meet your requirements, by Orson Scott Card.

My all time favorite fantasy series is not about a wizard; the Temeraire series, by Naomi Novik. Another series that I've enjoyed a lot, also not about a wizard, is the Queen's Thief series, by Megan Whalen Turner; this one has more books (at least one more) coming out.
Michael Moorcock's Elric Saga might work

Steven Brust's Taltos (Dragaera) novels have him as an accomplished witch, and mediocre sorcerer in the first book (starts medias ras) and his learning sorcery and witchcraft isn't too belabored

The second half of Zelazny's Amber books, which revolve around Merlin have him start at pretty much the zenith of his learned magic capabilities
Only a few of the Dresden the files, like his style and humour, but very predictable, so gets a bit boring. Non of the others.
I'm also recommending the books of Christopher Stasheff

https://www.fantasticfiction.com/s/christopher-stasheff/
Like a lot of series, the books do get better. So give it a go again. What book are you up to in the Dresden Files?
But unless you know what order to read the Eternal Champion books in, it can be a right mess. Any chance you have a reading order?

Another really good series is Discworld by Terry Pratchett. The first two books feature a really interesting wizard. There are witches and other things in the series. It's really fun and funny even though not all the stories have magic in them.
An interesting question. I can't think of one off-hand.

In my experience, if the main character is a magic user, the series starts off with them learning how to use their magic. If the magic user is experienced already, they're never the main character.
Kevin Hearne - The Iron druid chronicles







Hounded - Book one in the New York Times bestselling Iron Druid Chronicles series

Atticus O'Sullivan, last of the Druids, lives peacefully in Arizona, running an occult bookshop and shape-shifting in his spare time to hunt with his Irish wolfhound. His neighbors and customers think that this handsome, tattooed Irish dude is about twenty-one years old - when in actuality, he's twenty-one centuries old. Not to mention: He draws his power from the earth, possesses a sharp wit, and wields an even sharper magical sword known as Fragarach, the Answerer.

Unfortunately, a very angry Celtic god wants that sword, and he's hounded Atticus for centuries. Now the determined deity has tracked him down, and Atticus will need all his power - plus the help of a seductive goddess of death, his vampire and werewolf team of attorneys, a sexy bartender possessed by a Hindu witch, and some good old-fashioned luck of the Irish - to kick some Celtic arse and deliver himself from evil.