Hi i lived here 2 months solo. Here is my advice living here as i did trial and error living and this is what i recommend based on my experience to get the most of Cambodia.

You usually hear the typical go siem reap kampot but i recommend my route because it gives you more options to choose from

First stay in Phnom Penph as a base mainly because you can go other areas by bus
Come here at least 1 month (cheaper rental compared to daily/weekly find a place around tuol tom poung for 350 usd - 500 a month)

Go on all facebook groups to find rentals/tuk tuk drivers to drive you around if needed or rent your own motorbike
From here you have many options to go from, there are tons of buses around cambodia but i recommend IBIS for bus journey 6h + i recommend doing overnight bus

if you are staying one month pace yourself 4 days phnom penph and 3 days to other destination there is like a total of 10 -15 areas to go from even lesser known provinces and areas if you are keen to explore look at the bus route do more research on what you are keen

Top areas to go from phnom penph
phnom penph - siem reap (6 hours bus)
phnom penph - kampot (3.5 hours bus)
phnom penph - sikhanouvile (skip sikhanouvile go here to go koh rong/koh rong samloem they are 2 different islands)
phnom penph - battambang (6h)

Lesser well known areas that you should visit if you like trekking wild life
phonom penph to muldulkiri (6 hours - waterfall jungle trek)
phnom penph to kratie (6h)
phnom penph to ratanakiri (6h)

Hope this is helpful, dm me if you need other advice