You can explicitly create an additional undo point via <C-G>u
in insert mode (:help i_CTRL-G_u
).
Because that is tedious, I would recommend to build mappings that trigger this. For example, when completing a sentence:
inoremap . <C-g>u.
Or when inserting a register:
inoremap <C-r> <C-g>u<C-r>
You can also create one when pausing briefly, through an :autocmd
:
autocmd CursorHoldI * call feedkeys("\<C-g>u", 'n')