The best arguments against proportional representation seem to be: (1) no one wants that many members of Reform UK in the Commons and (2) every election would ultimately come down to the Lib Dems (maybe plus a regional party / the Greens) deciding whether Tories or Labour get to form the government.
Setting aside that Scotland, Wales and NI are not "regions", the Tory and Labour parties are already unwieldy coalitions of disparate interests forced into coalition by the FPTP system, with all the infighting that entails. The only difference is that the coalition exists prior to election.
Sequential proportional approval voting is both proportional and favors parties closest to the center of voter opinion. No idea how many seats REFUK would win under it, but it's probably fewer than what they would get under party-list.
To your 2nd point: In Germany, we have proportional representation, and in the last 25 years, the Greens were only 9 years in power, the Liberals 7. Over this period, we had 4 different combinations of parties in government. With parties like SNP, you probably would have even more variety.
