I really prefer the 5e proficiency bonus/bounded accuracy over ad&d. But I miss the feel of ad&d.
I'll respond to a few things below, but don't discount what you do behind the DM screen. My 5e games definitely have an OSR feel largely due to the danger levels of my encounters, embracing player choice, and focusing on challenging the players just as much or more than challenging their character sheets.
3. replace ASIs with an additional save proficiency with each occurrence (so most get all saves by level 16)
There's a problem there with #3, which you took note of below. By 16th level, most classes will have attained proficiency in ALL saving throws, so the 19th level ASI (which you've proposed reinterpreting as save proficiency) serves no purpose. It's not insurmountable, but you'd need to address it with some tweaks.
This issue is magnified for certain classes. Fighter, Rogue, and Monk in particular.
By 12th level the Fighter would attain prof in all saves, so the 14th, 16th, and 19th ASIs serve no purpose.
For the Rogue, the 15th level Slippery Mind (prof in WIS saves), 16th and 19th ASIs would serve no purpose.
And for the Monk, the 14th level Diamond Soul feature (prof in all saves) is invalidated.
4. gestalt multiclassing (no bard/monk/paladin/sorcerer) instead of 5e multiclassing
@Sword of Spirit had some good ideas for how to approach gestalt multiclassing in 5e over here:
D&D 5E - Gestalt multiclassing, how's the balance of this proposal
5. no warlock? (Maybe letting sor/wiz pick up the subclasses if desired)
I think more than banning warlock, the issues that the warlock class magnifies are ones to pay attention to. Namely, (a) spammable cantrips (actually I just posted on this in another thread, but
Beyond the Wall has a great approach), and (b) certain eldritch invocations (and rituals for that matter) allowing spamming things like
detect magic.
If you can address (a) and (b), which seem to undermine the AD&D "feel" more than the presence of a warlock class, then you should be good to go.
7. Maybe remove charisma skills?
There's a happy medium there that works well for OSR games. It relies on two rules, and a bit of creative thinking.
First, as DM you decide whether a check is even possible (or whether a task is an automatic success), not the players. So talking down a hostile warband of hobgoblins might not be possible with some flippant role-play and a skill check. It's just not going to happen.
However, if the players have done the legwork to know that Khagar Din the hobgoblin warchief is allegedly dead, but the body was never found,
then a clever player leaning on this information might unlock the ability to make a Deception check to trick the hobgoblins. Good enforcement of this fundamental principle of 5e really helps to evoke an old school feel.
Second, definitely read over DMG page 245 about Conversation Reactions with DCs based on a creature being friendly/indifferent/hostile. It's not perfect, but it actually captures the gist of 1e style reactions fairly well. I generally use it as a touchstone on my DM screen and combine with old school reaction tables as needed.
Finally, when I prepare interaction encounters, I sometimes include a bit of structure in the form of questions the NPC might ask, and answers/lore/aid they can offer the PCs. I've found this to be a very organic merger of the mechanics with the roleplaying so it's
almost seamless.
Any ideas on how to fill out asis after the first 4? Rogues now have a dead level at 15 and monks at 14....
The 1e answer would probably be followers & strongholds.