In PF1, it was the case that the party level might be considered higher or lower according to the party's total wealth. However, it seems that PF2 might prefer a different approach based on how it suggests adjusting encounters for disparate party member levels (pg 508).
For example, if a 1st level party begins with 300 gp total wealth (in addition to starting assets), should they be treated as 2nd level? As 3rd? Or should party size instead be considered as having some number of extra members and encounters continue to be designed for level 1 characters?
What's the right approach for this?
I find using wealth very difficult to judge for encounters. A bunch of first level characters with a single high value item isnt as powerful as each with a appropriate amount so be careful. Over the 30 years of Dming ive learnt that items are worth less than the characters ability to play the system or rules especially at low level.
My advice would be to simple start off simple encounters first in "random encounter" style play and see how quickly your monsters dissapear. Once they reach the "real" dungeon you can adjust a bit as you go. All Dm's are aware that HP are simply guidelines for you to adjust when your favourite monster needs to survive an extra round for effect. The player who knows how many hit points a monster has doesnt do well in most campaigns!
However to try and answer i suggest you take the average GP wealth of the party whilst taking away each party members charged items such as potions and scrolls (not Staffs!). That way it gives a more balanced approach. Each party member above 4 i would add 2/3 or a level rather than full as one extra person doesnt make them all 2nd level as there is a huge difference at low levels. Try not to take into account situational magic either (Rod of Cancellation etc)
A much larger percent of PC progress in PF1 was through money, so altering that had a larger impact than in PF2. You could fiddle with money and had to monitor it perhaps too much as a GM to get the right balance.
In PF2, to make a large impact you'd need to be far more generous or severe to make it worth altering encounters by a whole level.
It's 14 levels between a +1 attack item and a +3 attack item, during which all classes have increased by +14 + proficiency increase(s). A 14 level difference in wealth to get that +2 difference would be monumental! Controlling XP is what controls power levels in PF2 because most abilities are hardwired into level progression via proficiency.
A 20th level warrior in PF1 would struggle to escape a prison of mundane guards if using only the weapons & armor they found on the way. You could lose many levels in power w/o magic. They'd hardly be legendary.
In PF2, they'd auto-crit their way out, barely scratched w/ their innate AC from level & unarmored proficiency, innate stat boosts, and so forth that's hardwired into their system. It'd be an easy escape, much like you'd expect from a legendary warrior.
Much better approach I think.
I guess a good example of mechanics is that an NPC w/ PC wealth could move up a CR, and that was often the go-to bump for AP bosses.
In PF2, NPCs built via PC methods can get the same amount of wealth already without threatening the system.
Heck, in PF1 they advised to beware of the effect regular-equipped NPCs could have because they had more money than a standard monster just to keep competitive in battle.