I've toyed around with something similar myself, but if you want it all comparable to 1E you don't want a 50/50 chance for learning 1st level spells, etc.
Also, it should be a spellcasting check (so the PC can add proficiency modifier) instead of just an Intelligence check, unless you want it to be harder to learn spells, which you might...
shrug For "school-specific" spells, I would double the proficiency bonus.
Here is some math for it:
With DC 15 + spell level:
View attachment 125716
With DC 10 + spell level
View attachment 125717
Since 65% was more typical in 1E, a DC 10 + spell level Intelligence (spellcasting) check works better IMO anyway.
Also, here is the DC 10 + spell level with double proficiency for "school-specific" spells:
View attachment 125718
So, specialty spells would be very likely to learn.
That's some mighty fine number crunching. I hadn't gotten further than the theory of it, since the likelihood of running a full fledged 5e campaign isn't high right now... but will definitely think more if I do.
My experience is almost exactly like yours!
1. I loved ... LOVED Weapon v. AC. It really made the different weapons "sing" in terms of differentiation. But it was too finicky and limited, given the sheer number of monster encounters. So that was dropped. Begrudgingly.
I use most of the fiddly elements of 1st ed, for many of the reasons stated above - once you start removing things or easing restrictions the seams start to pop.
With Weapon type Vs AC the thing to understand is that it doesn't apply to ALL ACs, only to artificial armour worn by creatures. It's easiest to shorthand it to 'Natural Armour' in which case you ignore the modifiers entirely, and 'Armour Type' in which case you use the values according to your weapon. The extra calculation doesn't slow things down much (especially of the players have their #s on the sheet in front of them) and certainly adds a layer to weapon choice.
I've seen threads where people try and apply it broadly to natural armour types as well, but that just seems like unnecessary extra work to me for a system that works as it is.
For Weapon speeds - again here's an example where keeping it all adds a level of nuance that makes combat fun and chaotic in certain circumstances. A tied initiative roll between combatants prompts a speed factor check, and if the faster weapon is twice as fast, (or 5 pts faster), then that attacker gets a bonus attack before their normal attack. A dual-wielding thief with daggers who is behind his opponent will thank you kindly for this little rule.
Regarding the Polymorph spells - it's actually not as dire as you mentioned - Polymorph Self doesn't require the system shock, though a wizard can't use it on a friend, only themself. Polymorph Other is meant as an offensive spell, so there's no friendly version, thus the SS roll.
But the Wand of Polymorph allows for both options, and can be used to safely polymorph an ally without requiring the SS roll.
As to other restrictions, hell yes, I definitely keep them. I don't get complaints about low level limits for demi-humans, because the trade off it worth it when you look at the fine print.
Elves and Half-elves can be Fighter-Magic users and cast in full armour. If you want to favour the wizard an Elf has a base cap of level 5 as a fighter, they have a base cap of 9 as a Magic user, if you want to favour the fighter a Half-Elf has a base cap of 6 as fighter and 6 as magic user. Those caps can, of course, be increased with higher stats.
Half-elves and Half-orcs are the only classes that can multiclass as cleric. One of my players made a Half-Elven Fighter/Cleric/Magic-user once and he was a dynamo. Or how about a Druid/Magic-user! Talk about potent.
Being able to enter a combat in full armour and still have access to all of your magic is immensely powerful in 1st Ed.
Of course, all of this is according to taste. I fully understand where some players will simply want a fast and easy system, but for me one of the reasons 1st ed is compelling is the nuance. The little details that seem cumbersome on the surface but add an otherwise unexpected layer of satisfaction to the experience.
And just so my ramblings don't go entirely off-topic - the 'restrictions' imposed on 1st Ed Magic users are a feature, not a bug. The randomizing of spells, the creation of a block list of spells you can't learn because you failed the roll, the cap on how many you can know based on your intelligence... all of that contributes to a satisfying game from the perspective of the challenge. All of those things make you think about your spell acquisition.
I genuinely think some of that would enhance the experience of wizards in 5e by adding another dynamic for the players to consider as their character grows.
Anyway, I'm rambling. Such are my thoughts on the issue.
