So your freezing is caused by spawning 1000 instances of the same actor? You could just space out the spawning by a few milliseconds by introducing a delay in your loop, which should prevent complete freezing. You can/should still async load a reference to that actor first though if you don’t want any freezing at all at the start, but you can just do that by having your spawning logic in a streaming level and just stream that level in (and your persistent level is basically empty except for some streaming logic).
And if you know how many actors you are trying to spawn, you could easily track the % of how much of the spawning is done and display that in a progress bar.