Unless my alloy was free or under $1/#, I wouldnt cast my practice bullets. So that is part of the "should I cast" question. Gear is still available. One thing about casting is it can be done pretty inexpensively. Lee 6cav mold, Lee sizing dies, Lee 20# bottom pour, maybe $150 all in. That is like 2000 comm cast bullets. Its the cheap/free alloy thing.
Your dive weights could be pure led or could be zinc or lead & zinc. 70# is about 3000 bullets so a good start but without add'l alloy, might be an issue. As noted, its a hobby unto itself. I enjoy casting & coating my own bullets more than reloading. Yet I still wouldnt cast my own practice bullets if my alloy cost was over $1/#.