I assume the Hobie 18' is an Adventure Island Tandem--they're great boats and sailing it is teaching you more than you think. Without a jib
to tack though they are much easier.
Handling a boat up to 40' is not particularly difficult, nor is ocean sailing necessarily worse than lake sailing especially here in SoCal.
I've been sailing for 20 years and was in the Navy
prior to that, so I've been doing this my entire adult life; that said, I do work and have a family and so I've been unable to take trips longer than two weeks, which has kept me confined to the "Ensenada to Pt. Conception" area here along with some guest sailing on the east coast
I think your best bet, given where you are in life and your plan, is to start bare-boat chartering for vacations. Keep your Hobie
I wouldn't spend any money
on an intermediate boat between now and your eventual 40' cat just to learn.
I've got that Hobie and while it's a piece of cake to sail and maintain, you are learning
what you need to know. I would mod it to add a hank-on jib
(look at what the guys on the Hobie Forums
are doing that way) so that you get the sail handling experience of tacking a 100%+ jib, and call it good.
Beyond that, take a few caribbean
vacations and bare-boat charter
to get some large boat experience. Make some friends on the coast that have boats and go sailing with them to learn the ropes.