Trying to understand the concept of seed phrase security and actually utilizing the crypto for purchases

I don't currently own any crypto, but I was curious about it, so I started looking into it in the hopes of understanding how it all works.

Here is what I have understood as of now (might be wrong) :

You have multiple private keys each tied 1 to 1 to addresses and one public key.
All of these are housed under a seed phrase (mnemonic phrase) that acts as the key to claim ownership of crypto "shares" (ledge ownership) on the blockchain.

People can send me funds to a one time address (or my public key?) that will be added to my position on the blockchain.

I can send funds to people using one of my throw away "private keys" that are housed under my seed phrase

Crypto exchanges are like unregulated banks that have minimal safety reserves and are highly targeted by hackers and are subject to the exchange managers miss using their custodial position.

Hot wallet are programs or apps that serve as a "key ring" to allow you quick access to your ledger position on the blockchain. While they are not subject to exchange managers abuse they are still vulnerable to hackers due to the seed phrase being exposed to the internet.

Cold wallets are physical devices (like Trezor) that can generate and maintain a seed phrase offline by instead providing "signatures" for online transactions that are being requested by an app or something. This requires you to physically connect the cold wallet to the device with the app in order for it to retrieve the "signature" and complete the authorize request.

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Please correct me if I am really off base for the above but we'll considered that I have understood the basics above in order for me to expand on the following questions.

Where I am at a loss is in terms of air gapping and protecting the entropy of my seed phrase while simultaneously using crypto to purchase goods and services with out being forced to lug around a physical cold wallet device or tapping into my holding after a long accumulation phase.

From a purely accumulation stand point, I could set up a Trezor "cold wallet", have it generate an offline seed phrase and get a public key or adresse (I'm not sure how, without going online) and start sending funds to my position on the blockchain.
At this point my security level is seemingly very high, as I have a physical encrypted device with my, lets say my 12 word seed phrase that has never been exposed not he internet, that I could also memorize and load into a hot wallet in case I lose my Trezor device.

Where I am lost is in terms of lets say I want to protect my holding / seed phrase all the while using my account to send funds, purchase things, etc.

To effectively do that, I would not always want to be forced to have physical access to my Trezor (I don't want to open a safe every time I want to make a purchase or transfert my money), or load my seed phrase into a hot wallet effectively exposing my whole account to the internet/world and having all my holding potentially wiped out.

First question:

For small amounts of money this is not a issue but lets say I have 100K under my Trezor generated seed phrase, my Trezor device is in a safe somewhere and I want to send 100$ to a friend across the table from me. My only course of action would be to expose my seed phrase to the internet in order to complete my 100$ transfert, effectively expose the remaining large balance to possible theft?

Second question:

Lets say I can no longer retrieve my Trezor device (war, natural disaster, etc.) but I have my seed phrase memorized, my crypto value is 1M USD.
When I get to my new "stable" location and want to deploy my capital.
How would I go about doing that without exposing my 1M USD to theft.
How could I partial dip into those funds (probably can't safely convert 1M worth of crypto to anything in one go) without exposing my remaining balance ? and since I am now without a Trezor device, I have no way of re-securing my remaining balance anew by setting up and transferring my remaining crypto assets under a new secure seed phrase ?

TLDR : I'm looking to understand how to maintain non-custodial control over my crypto assets, while maintaining a high level of security and being able to use this currency at the same time to purchase things or if I am only in accumulation territory for several years, how do you go about tapping into it at the end without being severely exposed to theft due to a conversion bottle neck at the end point when slowly divesting out of this asset class.

I hope what I ask makes sense and thank you for your replies.