Best way to manage a minor windfall (£400k) without paying into pensions.

Britain seems to be in pretty terminal decline, and you can't trust pensions anymore. So any advice on how to manage this would be greatly appreciated.

My wife is about to inherit ~£300k from an uncle, and we’ve also got ~£100-120k in private shares (by my reckoning) that should IPO next year. That leaves us with around £400k to think about. We're both 34 YO. We own a home in Scotland that we don't intend to move again, about £500k mortgage on a house worth about £700k.

My current thinking:

• Use both of our ISAs and our son’s Junior ISA each year (£20k + £20k + £9k = £49k total)
• Rinse and repeat annually until it’s all inside ISAs
• Keep the rest in a medium-risk accumulating fund in the meantime
• Withdraw £49k each year to feed the ISAs

A few questions I’d love advice on:

1. Is this ISA strategy sensible, or are there smarter ways to phase it?
2. Should we sell the IPO shares straight away (and pay tax), or sell in a phased way…
3. Are there other long-term tax-efficient options worth considering (I’ve avoided pensions so far as I don’t love the idea of money being locked up, but open to being challenged – I’m a higher-rate taxpayer in Scotland, so tax savings might outweigh the lock-in).

Goal is long-term safety for family and sensible achievable growth, but without over-complicating things. Any thoughts appreciated!

Edit: we've about £20k in debts we'll clear when we get the money in, and a monthly income about £8k