Turns out on part 2 you can remove on access rather than after a full sweep of the grid, which cuts down the number of iterations you need to do about 1/2 sometimes 1/3 (depending on input).

import itertools as it
from pathlib import Path

import numpy as np

cwd = Path(__file__).parent.resolve()


def parse_input(file_path):
  with file_path.open("r") as fp:
    grid = np.array(list(map(list, list(map(str.strip, fp.readlines())))),
                    dtype=str)
  return grid


def solve_problem(file_name, single_attempt=True):

  grid = parse_input(Path(cwd, file_name))
  nr, nc = grid.shape
  nacc_total = 0
  stop = False

  while not stop:

    nacc = 0

    for i,j in it.product(range(nr), range(nc)):

      if grid[i,j] != '@':
        continue

      if np.count_nonzero(grid[max(i-1, 0):min(i+2, nr),
                               max(j-1, 0):min(j+2, nc)] == '@')<5:
        nacc += 1

        if not single_attempt:
          grid[i,j] = '.'

    nacc_total += nacc

    if nacc==0 or single_attempt:
      stop = True

  return nacc_total