Encoding: Insert keyframes at certain positions
Is there a simpler option using -force_key_frames? Unfortunately, this option only seems to allow inserting key_frames using an interval

Not so. To force keyframes for frames 45, 99 and 154 where first frame has index 0, use

-force_key_frames expr:eq(n,45)+eq(n,99)+eq(n,154)


You may need to escape the commas, depending on your shell.