While I'll admit I'm a bit leery of adding early DnD like "spellcasting" to SWSE I have given it some thought over the years although nothing I've put down to paper.
Now before we even go there I'm sure the most basic comparison of Spellcasting is to the use of Force Powers where a "spell" would be the equivalent of a Force Power and that you'd probably learn spells the same way. Now if you are looking at how "spell level" might work into this here is where I'd point out the various DC requirements certain Force Powers have; a power like Surge that has a minimum DC 10 for any effect is your "1st-level" spell or even a cantrip as it is something you expect to "cast" reliably early on with that +5 bonus to UtF that having the skill trained gives you. The spell's power increases as the user gains ability (UtF modifier goes up) which keeps it relevant. Other Force Powers have higher DCs for minimum effect and in many ways those reflect a "higher level" spell because for all of its potential use the DC 25 minimum that the Phase Power requires means it's something that most low level "users" really shouldn't consider learning. Additionally some of those powers are rather limited (even if highly useful) with the minimum DC and may need a DC 40 check to use to maximum effect which means a high level character who may have some specialization in it besides that. All of this is simply to show how "spells" could be reworked as Force Powers and that mechanic would be used.
Now in a departure from how SAGA works I have wondered how making a new class where levels in the class open up spell casting levels or a class where there is some "Spellcasting talent tree" where you spent talents to open up new spell levels would work. Now looking at what you have already maybe there is some feat that opens up spellcasting ability like Force Sensitivity opens up those aspects of the game. This "Spellcaster" feat would allow training in a "Spellcasting skill" but untrained uses would be your "little" things much like you could do with just Force Sensitive; training the new skill would open you up to the basics. Now after that I'm thinking about making a PrC (which could require less than 7 levels before you could enter it) and as a class feature it could open up new spell levels; opening a new spell level every other level would mean +5 levels in the 10 levels most PrCs last. After that you could then have your "Archmage" class.
Now I'm not entirely use how the spellcasting would/should actually work although I do expect a number of parallels to how UtF works. Now even if you want Divine and Arcane spellcasting you would still only need the one (plus more advanced version) spellcasting "class" where you are going to be required to make selections on which your focusing on each time you'd get a new level of spells you could use.
Now all of this is theory. While I never got into it I wonder how 4e (and maybe even 5e) handled spellcasting and how well, or even if, that might work in a SAGA type game without completely throwing off everything else. At least in the earlier editions of DnD it's spellcasting that radically threw off character balancing as "fighter types" could rarely keep up with full casters. As long as Skill vs. Defense is addressed I don't find Force Users to be so much more overwhelming than other characters of the same level especially if presented a variety of situations.
Some good points that I hadn't fully considered yet (especially about the high DC Force power effects). I was at this point considering spellcasting as it works in 3e/Pathfinder/Starfinder, and how that would work with SAM mechanics, basically putting the spellcasting to a skill as you described (using SAM mechanics). So far, I did imagine a single spellcasting class (the Mystic, basically) with multiple talent trees to choose from, which would determine the path they'd be taking. The difference would likely be the casting stat (Int for Wizard-types, Wis for Cleric-types), which would be determined by the initial talent tree. I'm thinking there would be something like an 'arcane' tree and a 'divine' tree, which would then lead to other trees depending on the choice. The idea you mentioned about a special PrC would likely have special talents/secrets that would act like metamagic feats.
I've played 5e quite a bit, and ran a campaign or two (skipped 4e altogether); spellcasting is similar to the 3e games, though they've generally limited higher level spells to fewer slots, and made cantrips (0th level spells, which would be like the basic stuff one could learn just by having the "spellcaster" feat) into your primary reusable attack spells that scale with level. Basically, that first level fire bolt that deals 1d8 damage scales up to like 4d8 at higher levels, and can be used at will throughout the day without expending slots or spell points to recharge. There are actually some potentially interesting options in 5e that could be implemented, such as how the sorcerer can manipulate magic via spell points, and how the warlock only has a couple spell slots, but they recharge after like a 15 minute break. They also have the paladin's spells getting burned to power their smites, which add extra damage based on spell slot used.
Aside from multiple SW campaigns, I've done some little bit of homebrew with Saga (mainly running a couple of Fallout games, with one using homebrewed X-COM enemies, as well as homebrewing some Zerg that I threw in as Sith mutations during one campaign), so after a bit of a refresh, I'm pretty familiar with the system.
Glad to see I'm not the only one homebrewing Zerg and X-Rays 
Now as for your original question and concept, this might serve you well. It basically uses Saga mechanics with some 5th Edition ideas fit to a Saga chassis for magic.
Saga D&D
I've not done much converting Saga Force powers into spell slots, but I have make a few magicians and spell-slingers using Saga rules and no real homebrew changes. You might be better served by making spell chains (requiring certain spells to be mastered before magic-users can access the next level of spell), as the current setup of Saga Force powers is more similar to the concept of psionics than magic.
I figured there had to be others trying out Zerg and the X-COM aliens, hehe.
I'll have to take a look at that; seems like some useful i information there; thanks for sharing!
I had actually considered a psionics class that used some of the Force powers, but I'm still too focused on working out a way to handle traditional magic to worry about that at this point.