There is a current thread on the sleep board about nap extending - this is another 6+ month old and might be useful in terms of what the OP has done to achieve cot naps for the daytime.
Is nap length and self settling developmental?
Nap length is, but you are about at the age when baby should be able to learn to extend naps. This happens somewhere around 5m-7m old, give or take.
Self-settling is tricky to answer. Much depends on what you define as self-settling? Babies cannot go to sleep like an adult (tired, lie down, close eyes, sleep) until they have the emotional development at around school age - so 4 or 5 years old. Until then babies need something to provide the comfort and security to be able to go to sleep. That said, the something can allow for independent self-settling - as in baby can use their sleep trigger to go to sleep by themselves.
The something babies use as a sleep trigger can vary wildly. It might require a parent - rocking, bf to sleep, movement and so on. If you establish this as the sleep trigger then baby won't just grow out of it without you teaching a new way to sleep.
do i focus on extending naps as much as possible or focus on getting him to settle then hope sleep times improve?
I think both are tied together. If you can start teaching baby to settle to sleep in the cot, you can use the same methods to try and keep baby asleep for longer when he first starts stirring awake.
You can still go out, but when he's awake.
Daytime naps are usually now 30-40 minutes... He gets tired after 2 hours, sometimes 1 hour
This is a usual routine for when baby is on short naps. I'd probably be aiming for a nap about 90minutes after waking while naps are short.
But you could encourage baby to extend the naps so that short, frequent naps changes to 2 or 3 longer naps per day.
I favour the firm hands to settle a baby to sleep in the cot. Feed baby, settle him and then lie in cot. You sit/lie right next to the cot and a firm hand across chest/shoulders helps sooth baby with your presence. A second hand over kicky-legs also helps baby calm and sooth. A tightly tucked in blanket also does a similar job.
I use a dummy, so firm hand on chest, re-inserting dummy as required and the occasional finger-pat and/or shushhhhhhh as needed. The idea is to teach baby to be still and calm to go to sleep where he will wake up - in the cot.
Similar tactics can be used to extend the nap - if you get to baby quick enough so he doesn't fully wake.
Can i do any,hing to shorten the evening put down process?
As above to establish going to sleep in the cot. Also, establish a sleep trigger that can be used by your DS independently to you. I favour dummy and comforter toy for this. It means over time baby can learn to gain comfort and security from dummy and blankie (as we use) rather than needing me.