2. In Ledger, you need Ledger Live. In trezor, I don't know because I haven't used it but you check their website -
https://wallet.trezor.io/#/You can use ledger live to operate ledger nano, but electrum wallet is also built as hardware wallet extension, you can operate your hardware wallets on electrum, be it trezor, ledger nano and many others. Electrum is all in all wallet.
Is there another more definitive, secure method of accumulating BTC for long term holding?
Just use a wallet that you hold the private keys like Electrum.
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For accumulation, cold wallet storage will be good. Electrum cold wallet storage is fine while another device needed for watch-only, you can have a paper wallet, and so ways to create airgapped wallets like hard drive wallets. But for ease of use, cold electrum wallet with watch-only wallet and hardware wallets are recommendable.