While maintaining large funds on an airgapped device and the rest on a HW is a good option as well, why would holding everything on the HW be a potential problem?
Because HWs are not perfect; because with the right malicious wallet, even HWs can be fooled and money lost.
I think the level of knowledge necessary in order to use both at the same time is much higher than simply using a HW
My point was to "feed" the HW from cold storage. That should happen very seldom - once per year or less. But yes, it needs higher level of knowledge (and I stated that one has to know what's he doing).
The chances of messing up with an airgapped Electrum are higher imo as soon as you don't have that much of an idea what you're really doing.
I've played a little with Tails OS and it was really simple for me. Electrum was there and I only had to make sure it's indeed offline.
Of course, I followed as good as I could Electrum's doc on cold storage.
I've been personally using my Ledger mostly and some paper wallets every now and then, but I trust my Ledger more than any other method of storage and never made even one little mistake through it.
The big question is: how you handle the paper wallets? I know stories of people losing their funds exactly while they were trying to fund their HW.
How reliable are HWs in terms of holding funds for long term (maybe 5-6 years into the future)? Like HDDs/thumb drives failing to start or get corrupted, do HWs have any similar issues???
That's something you really have nothing to worry about. You write the seed down to paper as instructed and the seed can work with most modern wallets. But I didn't read yet stories about HWs broken.
And if it will just "forget" your info, you can easily re-init from the written down seed.