"That doesn't, IMO, nerf casters."I understand why you would recommend this, but I don't think it is overly harsh to say that they must have either the component or the focus to cast the spell. The material component is actually a focus, a way to help you get in touch with and manipulate the weave of magic. An arcane focus, druidic focus, bard instrument, or holy symbol allows you to take that deep understanding of the magical nature of the material component and focus it through this spellcasting focus. It is a shortcut only available to dedicated spellcasters who have spent much time deep in study and practice.
In short. If you have a spell-casting focus (and are of a class that has spellcasting focus as a class ability), you can cast the spell.
If you do not have a spell-casting focus, you can use the actual material component.
My only change is: in order for your spellcasting focus to replace a material component for casting purposes, you must first have studied and practices the spell with access to the appropriate material component.
That doesn't, IMO, nerf casters. They can hide multiple component pouches on their person if they are worried about losing both their focus and components. They may also want to have some safe house, store room, hidden cache, or laboratory where they store extra components in case they lose the components on their person. This just drips spell casting flavor for me.
One other change I am thinking about now is that the spell caster can only be attuned to one focus at a time. But it is a special focus-attunement slot that doesn't count against magic item attunement limits. Otherwise, at higher levels, the caster can just have, say, an extra wand hidden on his person. I like the idea that if your wand/instrument/staff/holy symbol is broken, stolen, lost, you need to get somewhere where you can take the time and money to make or buy a new one.
On the other hand, allowing multiple foci can lead to some fun roleplaying.
DM: "You wake up in a dark cell, stripped of all your possessions and clothes."
WIZARD: I take out my wand and cast light.
DM: You are naked. All your possessions were taken from you. Including your wand.
WIZARD: I had an extra one hidden.
DM: Where would you hide it‽
WIZARD: ... Do you want me to draw a picture?
DM: Ugh. Yeah, right, you've been walking around the dungeon with a stick up your ass.
FIGHTER: It would explain a lot.
WIZARD: Look, it is written on my character sheet. Both in the equipment section and my character's background. Fecesious Stercus went to an all-boys wizards academy and to get around forgetting or being able to procure needed components to complete tests, they would sneak--
DM: OKAY! Fine.
WIZARD: Oh, I also have components for banishment and true seeing.
DM: *Sigh* Fine--What? True seeing‽
The must use comp to,learn depends on how easy comps are to get. If i want to learn at 3rd level the Aggie Scorcher are red dragon scales so common its likely i can find them?
If at 2nd level i want to add Tasha's but we are in the field, are there Tart Bushes?
Now, you may laugh at my Tasha example, but consider instead find familiar.
Now be a Rogue going Trickster at 3rd level in the field. No town nesrby but we leveled up. He is REQUIRED to pick his spells then and he will not get a chance to take another non-enchant spell until like 5 levels later.
So you put him in the quandry, pick a spell he wont be able to use for who knows how long, or pick a spell without comps and wait 5 levels for his familiar.
This gets back to the issue of changing the materials as part of a comprehensive change - like say "you only level during downtime at signigicant towns - week of training and work and cost/access."
Not a hypothetical, happened in the game i play. Waiting on level up info now but it looks like our Trickster just spent the entirety of his 3rd level without getting mats for his FF since we finally made it to the keep where we hope he can find the ingredients. His choice was to basically "waste" that spell because he did not want to wait until 8th and the special choice at 3rd cannot change.
He also skipped Tasha's because "no tart bushes" and cannot use focus so... How long do tarts last? I doubt he will ever take it even though its right up the trickster alley.
In practical terms, either the rule plays a role that is noticably problematic for material spells or its gonna be a lot of rule for little notice.
If its the former its gonna significantly impact spell choice.
If not, its likely gonna be not that much different so why bother? Is it woeth these rules for "when i level i spend screen time routinely gathering mu bugs and dust"?
We started our current game naked and equipmentless. Our whole first level was without our gear. My halfling could not cast light, but because i took all my other spelks as non-mats was otherwise not hindered. Still no light spell, no torches was significant to half the party.
But it could have been much worse.
Either was my list of prioritized spell candidates now lists them in order of material demand and availability.
Maybe you will find a sweet spot **without an overhaul** where there is "enough change and better flavor to,make it worth tracking" but not "so much change it negatively impacts long term play and limits spell choices."
But before that can be guessed at - how common are red dragon scales and how,long do tarts last in a spell pouch before going bad and smelly?