Interpret rhythms and recognize STEMIs. All rhythms.
Normal sinus rhythm, sinus bradycardia, sinus tachycardia, sinus arrhythmia, 1st degree AVB, 2nd degree AVB I & II, 2:1 2nd degree AVB, 3rd degree AVB, junctional escape, junctional tachycardia, ventricular escape, accelerated idioventricular rhythm (AIVR), ventricular tachycardia (VT or vtach), ventricular fibrillation (vfib), asystole, atrial flutter, atrial fibrillation (afib), multi focal atrial tachycardia (MAT), atrial tachycardia, wandering atrial pacemaker (WAP), AV Nodal Reentrant tachycardia (AVNRT, which people will usually call SVT), accessory pathway mediated tachycardia/AV reentrant tachycardia, atrial and/or ventriculcar paced rhythm, supraventricular tachycardia (SVT) when you can't tell which tachycardia it is, pulseless electrical activity (PEA) if is anything other than asystole/vfib/vtach that doesn't have a pulse
Some of those go together. For example, you might say sinus rhythm with a 1st degree AV block.
You should be able to describe features like left bundle branch block, right bundle branch block, intraventricular conduction delay, and premature beats. You might learn how to recognize electrolyte imbalance, particular hyperkalemia being the money maker. Maybe fasicuclar/hemiblocks.
Although taught, I wouldn't say it is necessary to recognize or calculate axis. Once you figure it out, it isn't hard and can be done pretty much instantaneous when looking at the 12-lead. Left ventricular hypertrophy might be a common thing to see and important to recognize for congestive heart failure patients and because it mimics an anterior wall myocardial infarction (looks like a STEMI, but isn't). I feel like other forms of hypertrophy are less important to know or are less commonly see, but might be observed eg right ventricular hypertrophy in an acute or chronic respiratory issue, left atrial enlargement in heart failure or associated with left ventriculcar hypertrophy.
A commonly recommended starter book is Rapid Interpretation of EKGs by Dubin Dale.
After that, I'd recommend The Art of Interpretation: 12-lead ECG by Thomas Garcia.
For practice, Amal Mattu ECG for Emergency Physicians 1 & 2 are good. Don't be fooled by the name. It is easily doable by paramedics. It's a book with a bunch of ECGs that you can interpret, practice without bias, and then look at the back for answers.
There is really no limit to how much a paramedic can learn for 12-leads. I feel like it is one of those things where everything is taught, but few of it is useful or seen. It can be difficult because some of the stuff is very specific, could be useful to know, but rare. Like recognizing fascicular VT (responds to calcium channel blockers, which is usually not what you give to VT patients...), RVOT VT which is adenosine sensitive and will convert, but these things are rare and nobody fault you for not recognizing it I think. Like even all the rhythms you learn, I mostly see the different sinus rhythms, a 1st degree block once a blue moon, atrial fibrillation, and atrial flutter or SVT or VT or vfib once a blue moon. Most of my arrest are asystole or PEA.