So is the starting beach an okay spot for me to base around or are there more defensible/ideal to migrate to?
Personally more of the opinion that it doesn't actually matter all that much.
Unless you really blasted through the first boss and gathered just the mats for the flag to move, you should have got enough materials and such to setup a bit.
Raids, honestly, once you get the knack for them, aren't really an issue, just potentially time consuming, unless you increase the count too high, or the level (with items, not just moving) and got literally no real attack damage.
As such: Find a spot you like visually, if you do intend to build up a base and not just a storage to blast through the story.
Personally I do like the entrance to the spider mine( I mostly build there, but not for defensible reasons or similar, I just like the location, with easy access to iron and such) - but it honestly doesn't matter all too much.