Dungeons & Dragons 3.5 is not well equipped to describe the world prior to the Event. If you try to do so, you’ll be fighting the system every step of the way: 3.5 is extremely high magic, and expectations of high magic are baked into the system at every turn (most especially with how CR is determined, and by extension published modules). It certainly works better at low levels, but even at low levels, having some people play barbarians and crusaders and others play, well, commoners is not going to work well within the system. Anyone who wants to play a magical class is going to have to slog through a few levels of being useless dead-weight, and the other characters are going to need a very good reason for carrying that around.

Also, the claim that low-level spellcasters spend significant amount of time with slings and crossbows is not, in my experience, particularly accurate. Low-level spellcasters must use spells judiciously, which means, usually, they try to cast just one spell to give their side some major advantage (usually entangle or grease, sometimes sleep or color spray; other options are possible), and then allowing the mundane classes to take advantage of it. They might take potshots with a crossbow or sling, but that’s mostly just for something to do: they’ve already done their major contribution to the battle.

What you propose eliminates that contribution to the battle.

Finally, the world of Ishtar pre-Event seems to be lacking much in the way of major conflict. The players might be sent on your typical rat-killing quest, I suppose, but mostly this is not a world where the rules of Dungeons & Dragons are appropriate: those rules are really designed for dungeon-delving and dragon-slaying. They have little to offer a world of prosperity and comfort.

My solution would be to run the game in a different system entirely, prior to the Event. Something rules-light and narrativist; these seem more appropriate to the tone of pre-Event Ishtar, and the issues of magic need not even come into play. Where Dungeons & Dragons is ill-suited to this, Fate would be excellent. In fact, considering the relatively-brief time spent pre-Event, I would probably go with Fate Accelerated Edition.

I would have the players still pick their post-Event D&D class and ability scores while building their pre-Event Fate characters. I wouldn’t have hard-and-fast rules for what each class and ability score array means in Fate, but I would look for some commonality: someone with high Intelligence after the Event would presumably have had strong skills with Clever approaches prior to the Event.

Then the Event happens, and they become their 3rd-level D&D-character versions of themselves, complete with magic as appropriate.

I can personally vouch for Fate being used this way; I played in a giant-mecha-style game, where the pilots were all Fate characters and the mechas were all D&D characters, and the mechas were supposed to enhance the natural abilities of the pilots, so this kind of commonality was required. It worked very well. For that matter, I’ve used Fate-inspired subsystems to replace D&D’s skill system wholesale, and that also worked well.

One difficulty here is the transition period. Spontaneous spellcasters have it relatively easy, as do divine spellcasters in general, but wizards (and wu jen) have an issue with regards to the spellbook. I would handle this through allowing them, at first, to use their spell slots on read magic. This spell is one “all wizards” can cast, and though that is usually something they simply have studied well enough to memorize without a spellbook, it could be that the Event itself imbued them with this ability.

I would then have the world itself provide the spells. Considering all of the physical and meteorological changes in the planet, which is likely caused by this new magic, I would also have arcane patterns appear in nature: rock formations, vines, even the rapids of a river. Temporary events, too, could work: the way a particular bush burns, the way tea leaves land in the bottom of a cup, and so on. Seeing magic in the world around them, and understanding it with their new read magic ability, allows these wizards the opportunity to begin to craft spellbooks. It also makes for a really fun way for you, as a DM, to offer special “loot” in the form of spells they can copy.

Since buying spellbooks already filled will be impossible, and they won’t be starting with a spellbook with months’ or years’ worth of spell-scribing going into them, I would also consider reducing the time required to scribe spells (but not other costs). Then again, wizards are so powerful that I might not, or maybe only for the lowest-level spells (cantrips, 1st- and 2nd-level maybe).

If ramping up tactics/strategy is what you're after I've got to suggest you take a look at "The Angry Guide to ... Combats", parts 1, 2, and 3. Some takeaways:

• Two of one monster and two of another is much more interesting--for all involved--than four of the first. Even if the CRs are the same any variation in abilities, ranges, &c. lead to strategic and tactical thinking, rather than toe-to-toe slugfest. On both sides of the fight.
• Make sure you have the objectives of the monsters/NPCs well-established in your own head before starting combat. When total annihilation isn't the sole objective on each side, things get more interesting. Remember, combat != encounter: combat is only part of an encounter.
• Terrain, terrain, terrain.

Secondly, there's an initiative variant in the DMG called Speed Factor (DMG5e p.270). This variant separates declaration and resolution of actions--don't worry, with practice it goes just as fast as individual initiative. One upshot of speed factor initiative is that the situation in-game feels like it's evolving much faster. Faster evolution=more chances to change tactics in interesting ways.

But combat doesn't just have to be about tactics vs. going toe-to-toe. I challenge myself to include more role-play in combat every session. (It's actually a written note-to-self on the backside of my table-tent.) Some of the actions this brings out:

• Intelligent monsters and NPCs call out directions and warnings to each other and address each other by name. They had objectives/motivations going into the fight, and every round the cost-benefit analysis changes.
• Throwing out a plot call-back or a plot hook can often get the PCs to put on the brakes and re-think their approach. Suddenly it's half the party saying "hold on!--these mooks might come in handy" while the other half's all barbarian-rage and seeking the Dark One's Blessing. What's more interesting than PCs in conflict during combat?