For instance, I have put hundreds of hours into working on the basic pass, and have barely begun to be even mildly proficient at it.
The problem with the pass is that it is easy to get too focused on the action and on the hands while practicing it - and doing it like that in performance is not always the best way. Remember the whole body during the practice. Find the off-beat, and do it in the transition from tension to relaxation.
Best moment to do it is when no one is looking - and those moments comes more frequent than what you would expect, if you just keep an eye open for them.
Try to practice the pass cold as well - meaning, do the pass instantly each time you pick up the deck.
I am a decent performer, but have a tough time with routining and transitions. Are there any standout resources that cover this area?
"Impro for Storytellers" by Keith Johnstone is a good start.
You will gain from thinking in plots and subplots. This perhaps sound difficult - but only until you see that everything is a story. That no presentation also is a presentation, and that you as a performer is a part of the plot.
Meaning - someone who does card tricks with no thought to presentation at all... that is the story of "Suddenly this weird guy turned up and interrupted our meal for no reason, and did card tricks until we asked him to go away".
That is a plot, no matter whether you think it is a good plot or a bad plot. Everything that happens is a part of the plot.
You can work from both ends. Easiest is to make a list of the tricks you want to do, then weave them together.
For example: Let's say you want to do a thing with coins, then something with a silk and then something with a newspaper. Three separate numbers without any internal connection.
A simple weave is to introduce the newspaper as early as possible - so its presence is established at the time you do an effect with it. Anything works - it can be placed on the floor beforehand, as litter, causing you to be slightly annoyed when discovering it. Or it can be a wrapper for the silk, or something else...
..so that feels familiar and logical when you later return to it.
Then, try to feed back what you've already done in the following pieces. If the coins vanished at the end of the coin trick - let them appear again in a later piece, weaving the first trick together with a later one. You don't have to think about motivation yet, just try to weave the tricks together. Establish before, and feed it back afterwards.
If you are familiar with graphics - view your act as a photoshop composition. A tiny stroke or touch on each layer. Each layer on its own might seem unimportant, weak or primitive - but with all added together, the result can be spectacular.
Then, bring yourself into the composition, your emotions and reactions. You are human, right - make use of that. Bring the whole bag of feelings into it - fall in love with someone in the audience (openly or while trying to hide it), try to impress her, notice the guy beside her, jealosy - try to make your rival look bad. Fail, sadness, find comfort in your art - end happy.
Bring phobias into it. Or do something against your will, while pretending that you enjoy it.
Conflict - resolution. Both in the small aspects and in the big. You struggle to find a card - and the sound engineer hates you, and put the wrong music on everything, and you must keep face; Struggling to find the card, while dancing Lambada against your will.
Contrasts. Slow/fast. Happy/sad. Easy/advanced.. anything works.
Then, try to adjust the methods to take advantage of your reactions and feelings. You slam your fist against the table in anger? Well, load something there. Etc.
Small steps, small additions and small changes. Many small layers - and the result will appear as something well thought out and complex.
Remember the whole body. Many are magicians only from the waist upwards. That might work in close-up, but not on stage. As Bob Read said "Good magic begin in the feet".
I know - this is fragmented advice. Still..
Also,I would like to start looking into performing professionally,
First of all, your plans must involve the paperwork. The more you love paperwork, forms and such things - the easier it will be. The business side is extremely important. If you don't have any love for the business side - find someone to handle it for you. I myself dislike those things, and everything is difficult because of my lack of interest.
Then, it's mainly a matter of getting exposure. If you are good, you will always have work, as long as you get exposure now and then.
Track down all kinds of "Open Stage"-shows, all amateur theatres (they often have small shows with odd artists), student organisations, local TV. Make sure that your name is mentioned everywhere you participate - to be advertised as simply "a magician" does you no good.
Well, I hope that you find anything useful in these ramblings. Trust your instincts though. Every single word I've said might be wrong, from your perspective. You make your own rules.