1. If anyone says 1-2 hours tops to complete this puzzle, they've done these before. For me, being the first time setting foot in ruins like this it was more like 3-4 hours.
3-4 hrs to complete the puzzle?

? You mentioned YT guides so I assume you seen those and yet you needed 3-4 hrs to do it? I mean unlocking the BP, not the mat gathering, right? That's 15 minutes, half an hour tops. I know as I did my first Guardian unlock not so long ago. Watched CMDR Exigious' guide, went there and about 20 minutes later I had the BP. No prior experience.
I can agree on longer times when you go totally blind and try to figure everything by yourself.
2. I arrived at this site when it was pitch black. Not good.
Any planetary hunting while on the dark side suffers from that. Not Guardian specific.
3. First time used a conda. Too big to park next to the ruins, I had to go back and get my DBX.
And that is totally not true. When I arrived at my site I saw Anaconda parked in the middle of the ruins. Parked, as the SRV lights under were lit and SRV was driving around the place. I tried to park my Krait which turned out impossible, first spot I found was 5 km away (!).
Anyway, getting smaller ship for the unlock is preferable.
4. Install PDT on the DBX to deal with sentinel missiles. This is a lifesaver if you're not used to srv combat like me.
Or you can dismiss your ship/park far far away. Then sentinel missiles won't bother you.
5. There are 6 pylons to activate. The time consuming part for me was finding their hiding spots.
Again, following some guide can really save you the trouble. Unless you really want to try it by yourself.
6. Getting around the ruins is annoying. I needed time to orient myself to best routes to each of the pylons and once I got familiar with that there is plenty of time on the countdown clock.
Yeah, countdown is annoying - introduces unnecessary stress to the whole engagement. Not only you need to get familiar with the site, fight off sentinels, you need to pay attention to the timer. No wonder that YT/reddit/forum guides are in such regard.
7. I installed 3 SRVs for contingency, in case of destruction, whatever, and I'm glad I did. The biggest issue is running out of ammo. Be ready to synth.
Haven't faced the problem, even tho I forgot the turret switch button while being under fire and I don't use SRV at all so I totally suck at driving that thing. Yet one vehicle was enough, tho ammo part is true.
8. Make sure you gather enough of the mats you need for the specific blueprint your going for. The FSD Booster needs 21 power cells and 21 tech components from the site itself.
And be prepared for abundance of cells and total lack of components due to RNG. While cells drop right and left you may get 3 times as required by the time you get half of components. At least it was my experience.