Have you had advise on a visa? Typically most people will go the DNV or NLV routes, but your husband's company setting up a Spanish arm will rule both of those out, I believe, so this would be quite specialist although not impossible.
We have gone through the DNV process and are just deliberating final choices. We aren't leaving anyone behind, so I can't help there - I'm guessing they're too old/too independent to be considered dependants on your visa, or they don't want to come? They'll be able to visit you for up to 90 days in every 180, although if they use that full time, they won't be able to go on holiday or travel anywhere else in the EU as they'll have used all their Shengen days. You can visit the UK, too, but if your goal is residency, you can only be outside of Spain for a total of 10 months in the whole 5 years, so you wouldn't be able to visit for long periods... roughly 2 months a year, maximum, and holidays to anywhere else would reduce that available time.
Do you speak Spanish? If so, it'll be easier. I'm guessing your husband does. Will his company bring local employment opportunities? It's supposed to be a bit easier to move in that case. The process will be laborious, they will want proof of all contracts, financials, business plans, etc.
Your A1/S1 only lasts for two years, and then HMRC will consider you permanently resident in Spain; which will mean you need a Spanish financial advisor, and the limits on things like savings and tax adjust. You'll want professional advice on all of that, but I'm guessing you'll need a Spanish accountant anyway - corporate taxes are a bit higher in Spain, but again, I presume your husband has researched that. There's usually a "start up year" for autonomo people, but I don't think that applies to companies.
You'll need all the usual stuff like ACRO checks and apostilled marriage certificates too, all of which must be dated within six months of when you apply.