Group would be 5 players, one forever DM who is glad to play for once.

Sounds like a rather large group, but the forever DM is going to make things way easier so it should be fine.

I am looking through different books to try and find an introductory adventure for us to feel out PF2e. One-shots were considered but did not feel like they would teach us as much about PF2e compared to a small adventure.

I would definitely recommend the beginner’s box, especially since this is your first time GMing.

Does anyone have some advice for a first time DM who is looking to bring their group from 5e?

Look through this so that you are at least vaguely aware of where things are

https://2e.aonprd.com/GMScreen.aspx

Remember that this system heavily uses keywords, so it may be a little overwhelming at first but after a few sessions you should start recognizing things and running will become way easier.

If you cannot find the rule for something don’t be afraid to make up a ruling on the spot and then figure out the actual rule later. Although do be a little bit careful with the core math at first as the four degrees of success system and 3 action system make small changes have a large impact. Once you understand the math then it is a solid foundation to change to taste.

I would recommend that you ask each player what makes their class unique, that way you can see if they understand their character. This is because in DnD 5e almost all of the work is on the DM, whereas in PF2e it is much more evenly divided.

Emphasize that reactive strike (This system’s version of Opportunity Attack) is rare and that the players should feel free to actually move around. Combat in PF2e is usually much more dynamic despite movement being way more expensive.

Rarity does not necessarily correlate to power, but instead to how common the item is in Golarion (The setting the books are set in). It is still good to get players to ask you for them as it helps keep the setting’s theme coherent.

https://2e.aonprd.com/Rules.aspx?ID=2530

I would recommend against allowing SF2e items until you are more familiar with the assumptions of the system as while they follow the same fundamental math they have very different assumptions. For example SF2e allows flight from lvl 1 while PF2e only allows it from ~lvl 8.

Emphasize when modifiers make a difference. For example if you have a bard/cleric and their +1 turned a miss into a hit or a hit into a crit, you will be surprised how often that comes up.

Don’t see actions specified by skill feats as “You need this feat to do the thing” but rather “This feat makes you much better at the thing”. For example: Group Coercion allows the player to attempt to Coerce up to 5 targets at once, instead of preventing a PC from coercing a group without the feat you can instead make it a Very Hard check.

https://2e.aonprd.com/Rules.aspx?ID=2630

Encourage people to aid as much as possible. What I do is get the players to describe how they are aiding, and if it is particularly creative then I will give it the easy or even very easy modification

https://2e.aonprd.com/Actions.aspx?ID=2292

Magic items in this system are fundamental to progression, so unless you are using Automatic Bonus Progression you should be careful to give enough gold per level. Don’t worry too much about going over as you can restrict the level the players get items by limiting the settlement level or forcing them to craft their items, or you could allow them to have the higher level (and therefore more powerful) items and increase the encounter difficulty to compensate.

https://2e.aonprd.com/Rules.aspx?ID=2741

https://2e.aonprd.com/Rules.aspx?ID=2656

Give enough time for retraining, or even allow players to retrain for free for the first session or two. That way if they really don’t like how their character plays they have a chance to change it.

https://2e.aonprd.com/Rules.aspx?ID=2447

Trust the encounter building guidelines, unlike DnD 5e they actually work for the most part.

https://2e.aonprd.com/Rules.aspx?ID=2715

Also avoid throwing PL+4 enemies at the party for the first 3 levels as the math is rather swingy before lvl 4 and it is hard to counter high level single target bosses with lvl 1 spells. More experienced players don’t have as much issue as they can manipulate the system better.

At higher levels it is not nearly as much of an issue as optimized parties can take on more dangerous fights at higher levels There is a reason that lvl 25 monsters exist.

https://2e.aonprd.com/Monsters.aspx?Letter=&sort=level-desc&display=table&columns=creature_family+source+rarity+size+trait+level+hp+ac+fortitude+reflex+will+perception+sense+speed

Even if you have the physical books I would still recommend having Archives of Nethys open on a phone/laptop for how much faster it is to find things.

Never assume that your players will do something, instead what I do is figure out what will happen if the players do nothing and go from there. If the players ignore the encounter you prepared then you can put it in your back pocket for later. This is coming from someone who largely runs more sandbox-y games so it may not be 100% applicable, but it is still good to keep player agency as your top priority. In addition never gate progression behind a skill check, no matter how trivial; instead you can have them ‘fail forward’

Don’t worry too much about messing up, your players understand that this is your first time GMing and will not care.

Finally and most importantly, remember that this is a game and your fun is just as important as everyone else’s.