So guards don't respawn / more come? my assumption was that until the alarms are off guards would just endlessly show up (makes sense in my head at least) so I've been trying to hit either the power building or the security building first
No, the purpose of alarms is to manage whether the whole settlement engages you or not and also is used as a mission condition for certain missions (do not raise the alarm or the mission fails). You generally don't want the alarms to be raised unless you're experienced and don't care whether the whole settlement is aggressive towards you but so long as you don't have a mission that prohibits the alarms being raised and can handle yourself well enough in combat (or can escape quickly) then alarms aren't that important.
still on the hunt for these fabled abandoned settlements that the youtube guides love to feature yet not reveal where they be - pity inara doesn't have a search for it
You can find abandoned settlements by checking system maps (but of course you'll need to have visited the system before to get this information) and looking for factions that are in the states of Civil Unrest and Infrastructure Failure (and possibly one or two others like those two). You can get this data from Inara, of course. Once you've found a faction in these states then you can check the POI list on the system map for settlements owned by that faction. They should all be abandoned.
Abandoned settlements
can be very good for gathering assets and some goods but it helps to have the settlement powered up so make sure you have a power regulatory handy along with at least a couple e-breaches to activate the power regulator housing panel and the alarms panel (and remember to take these with you after you exit your ship/SRV). You don't need to deactivate the alarms panel but you will likely alert scavengers if you don't. Also, be prepared to fight scavengers on arrival.
I personally think abaonded settlements are quite a bit of hard work for material gathering. First up is actually finding them (it's not too hard but way more difficult than finding an active settlement of the type you're interested in) and then needing to power them up if you want data. Whilst this is not a problem if you can find reactivation missions for these locations (not as simple as just finding the settlements, you'll need to find the station offering missions to them as well), it's a faff if you don't have this as you'll need to use up a power regulator or wait the annoying 3 minutes to take it back again (and I'm pretty sure doing this definitely triggers scavengers a lot of the time).
Finally, abandoned settlements have a lot of empty containers. You can get really bad dice rolls on this and end up with barely anything worth taking.
All this is why I learned to infiltrate and take stuff from
active settlements. If breaking the law is a problem for you then you can just target settlements owned by pirate factions, as their settlements are anarchy and no crimes occur (so you never get any penalty, even if you get caught). But being able to successfully take stuff from a settlement is pretty thrilling once you get into it.
I'd say the absolute best source for mats are the new AX missions (and they're easy enough to find) but you will need to get used to doing stuff in Thargoid space. That said, I've worked out that you can complete AX restoration missions with nothing more than an A rated Dolphin (
https://edsy.org/s/vwtVy1w) and a G1 Maverick suit (and if you want to kill revenants then just grab a G3 L6 RPG).
It's absolutely more risky but you get the best data rewards, way better credits than any other EDO missions and you get to take stuff from the settlements with no fear of scavengers turning up (and revenants are easy to kill and avoid imo).