IME Frozen cooked meat tends to dry out unless it's stored in some kind of liquid or gravy. I'd go with your original idea i.e. cut the big piece into two or three smaller ones, freeze the spares, cook one and eat it up fresh within 3 or 4 days, hot and cold.
I'm not imagining this, am I? People do slow cook large joints of meat & freeze it in slices for sandwiches?! I'm sure I've seen it before on here but now that I've bought a ridiculously large joint of beef I'm having a crisis of confidence! I could chop it into smaller pieces so we'd have enough for one meal & a couple of days of sandwiches & freeze the uncooked pieces. I had planned on cooking the whole lot & freezing the slices. Is that an awful idea?! I can't think straight this morning!