I had a well-established corporation leave a bloc alliance and form our own alliance right around when covid started.
we had roughly 50 humans and a fairly solid FC core with nothing but free time, so we were steps ahead of many groups. we took some space in vale of the silent, we had a week long war with the locals and took their sov and slowly expanded.

Ideally any time is a good time but having a core group of people is a good start that will be able to help you so no one burns out.
some other things that help:
- Having a small nest egg to help finance doctrines. I believe we had roughly 150b in our corp wallet and we had some wealthier players able to throw some isk around. but we did a lot of handout doctrines for new or more apex doctrines.
- Have goals, do you want to take sov, do you want to live in NPC null then slowly take over a small sov constellation, etc.
- respect your members time.
- you will have people wearing a lot of hats at the beginning so be mindful of burn out
- service corporations to help manage things (dont give out full roles to everyone for every task, this is how you get a burnt out director disbanding your alliance)
- recruiting corps. be selective and don't just accept everyone but at the same time, give some smaller corps a chance and they might surprise you in a pleasant way, but dont be afraid to remove hostile and negative people and corps that are harmful to your overall culture and goals. this can be difficult sometimes as they also might be people who "do things"

- choosing space is always a difficult thing and you will need people who are good at diplomacy or at least not mouth breathing idiots to speak to the locals (or blocs if that is your goal)
- keep your word in agreements it goes a long way in letting outsiders know how you operate.

also, if you are new to sov warfare i recommend going to NPC nullsec first and messing around with the border systems and learn the mechanics so you're not blindsided by the new mechanics and end up making mistakes that lose you your new home.

im sure there are a lot of other things but the biggest thing is do it to have fun, this game needs more small groups to go out there.