Constructive criticism between friends should be easy. If you can present it correctly there shouldn’t be a reason for them to stay mad at you.
I think the angle that I would go for is that a game of DND is a compromise between what the DM wants and what the players want. Your DM doesn’t want to prepare ahead of time, they might have a legit excuse they might not, I dont know. It’s not inherently a wrong way to DM. I myself come up with sessions sometimes hours before they’re supposed to happen, not every week but sometimes I have to. I can improvise though.
You and the other players want a more thought out experience, which is perfectly understandable. You, the other players, and the DM gather on whatever day of the week you gather, and try and play a game together for a few hours. The thing about those few hours is they should be worth it for everyone involved. You’re all probably busy people with a list of other things you could be doing other than meeting and spending a whole night on a not so engaging session. As players I feel like you’re not asking for much by wanting your time to be respected.
So wheres the compromise here? There are 3 obvious approaches I can see:
1) Acknowledge that they have extra time to work on the campaign and do it. Remember its a compromise, they dont have to spend the next 6 months banging out all the lore for the game, but having things ready more than 48 hours ahead of time would be a marked improvement. Its like writing an essay for school, sure they can bullshit it the night before its due and they might pass (they might even get an A). But if you write it ahead of time, its going to be honestly better and then in the time between when its ready to go and playtime they might be able to come up with small details to add to improve the session even more.
2) Premade adventures. They exist for a reason. There are loads of them, and there are plenty of free ones. All they need to do is read over them once or twice, probably at least 48 hours before game time minimum. Much less work involved.
and the last option, the one you probably don’t want 3) Acknowledge that they do not have the time/desire to create a campaign that satisfies the needs of their players and step down. Let someone else take the reins. They tried, which is great, but more effort was required.
Improvisation is an essential part of being a DM and it is something a person can get better at with practice, but you have to be real with yourself about your abilities. Even the DMs out there who are masters at improv put in the time to create and write things ahead of time. Bottom line is, respect your players time. Respect your DMs time too, but thats not what this ask is about.
Im sure a ton of people who read this are going to have opinions so be sure to check the notes on this post to get some extra ideas from people who may or may not be more competent than I am.