Firstly, welcome to Flyertalk DollyR, and please be aware that you are only allowed 5 posts within your first 24 hour period, after which you may post freely. So you may wish to delay or consolidate replies.

Business Class moves this into the realm of the possible. 5 First Class to MLE would indeed be very very unlikely.....

It seems from your research, you've got the gist of this. For a popular route, what you need to be doing is phoning up at T-355 days, at midnight GMT (1am BST) and securing the seats. You won't be able to secure the return at this point, but you are correct that you can repeat the exercise when the seats are released on the return date you want (though if flying Economy, you may have more availability anyway)

BA only guarantees to release 4 business class seats per plane (it used to be 2, but went to 4 in 2021), so there isn't any guarantee that you'll be able to secure 5 seats on a particular date, but the more flexible you are with dates, the higher the chances.

Now, the fact that your have 1 voucher each in 2 names complicates things, as these cannot be used on the same booking. You would need to have 2 separate bookings. One strategy would be for both you and your husband to phone up on the appropriate day, with one of you trying to secure 3 seats, and another trying to secure 2. If that works out, brilliant. If not, you may wish to fall back on securing 2 each.

Now if you only secure 4 then to avoid a "Home Alone scenario", you'll clearly need to make a booking for the fifth person (note, if a child is involved they can travel on their own booking as long as a relevant adult is on the same flight). You could make this booking using cash, or hold out and hope that further availability opens up for Avios seats, and secure it if it does.

Good luck.