How to tell your guests it’s totally okay if they can’t make it?

I don’t think there’s really a way to do it other than shrinking your guest list. I think it’s kind of weird to invite someone but hint in them not coming. I think given the circumstances it’s ok for you to just shrink your guest list.

Nothing I can think of that doesn't sound rude or that you're hoping people don't come. You really should have kept looking for a venue to hold 450.

You need to cut your guest list to fit your venue or find a venue to fit your guest list.

You need to put out the an apology to each person who got a Save, who will no longer be invited. And Truth fully say that in order to reschedule, given all the financial hardships of this crisis, you will be having far fewer guests, in a much smaller venue. Then send completely new invitations to those invited to the smaller event. Real reasons, people understand. And don't string along people who may making their plans around your event. And give the new, smaller guest list to parents, show it to shower hosts and such, so no one draws in those no longer invited. You would be up a creek if 450 people did decide they would attend, after missing so many things GS this year. Then after keeping the date open, they get no invitation, that would make people made. No one likes to apologise, but it is the only polite thing. And most people will be nice.

View Quoted Comment I totally agree. Just be straightforward with your guests about what’s happening and uninvite guests until you have an appropriately-sized guest list.

I don't think you can. I know this advice will go against etiquette, but I would only send out invitations to those you can accommodate even though some that were sent STD's will not be invited. If anyone asks, you will just have to be honest with them.

View Quoted Comment If you redo things on a whim, on short notice, that is not proper social etiquette. But when there is a compelling reason to cancel, and the following new event must have fewer people for whatever reason, budget or venue, then you do need to apologise and explain why you are not inviting the large group. But provided you contact those who got a Save who will not be included on the new date, and give the apology, it if proper etiquette. Good manners demands that you not jerk people around like interchangeable pieces. You should not drop ten people who got saves, so you can have a fancier cake, or invite different people for the same slits. That is rude. But this national emergency has affected long made plans, and a lot of people are hurting financially.
And in such. Circumstances it is allowable to make necessary cuts. As long as those not invited are politely told you are sorry. And that you wish you could invite them and everyone on your former list, but finances and the new venue size won't permit it. And those who have been apologized to, and assured that they are still valued friends, and the change was unavoidable, then need to be gracious and understanding, and not hold a grudge. That part of etiquette is as important as the couple imforming some guests about the change in a nice way. Etiquette is not so much exactly what you always must do. As that you do things in the most polite way when circumstances change, and both sides try to make it easy on the other. Less friction, less damage to friendships.
On these boards lots of people propose to do not very nice things, for not very nice reasons. And people call them out on poor manners. But standards of etiquette permit living in the real world, and making hard decisions in tough circumstances. As long as you do it nicely.