One situation is that I want my phone to show me the traffic congestion whilst I drive so I can alter my route dynamically. The screen locks every five minutes (longest setting available), but my commute is ~40 minutes.

That’s not an iPhone problem but a bug (or missing feature) in the app you’re using. Apps can disable screen lock while they are active with a single line of code. This is commonly used in video players, navigation apps, games, etc.

Depending how much you want to play around with it, you may be able to configure something with Shortcuts to get the desired behavior. I have something similar set up for screen brightness and media volume based on what WiFi networks I’m connected to or places I’m at.

Bet you they’re protecting themselves against battery life complaints. Apple Knows Best!

Not a custom lock option, but to make your life marginally easier:

There are no shortcut actions as of Mar’23 to trigger auto-lock. The closest thing I can think of (which I use personally) is this shortcut to quickly open Auto-Lock in settings menu.

So that would save a second… “Hey Siri, I’m commuting // autolock” -> the settings pop open. Source

From up that thread as well:

I always set my auto lock default to never. Then whenever I need the screen to lock automatically I'll just toggle low power mode from control centre.