You forgot an option for "I dm 5e but problems for other reasons" or similar I find that 5e monsters tend to be built for a small party of lower level under equipped (ie starting gear only) so that breaks down at higher levels (ie tier3 or late tier2) & with larger groups. Tools like flat DR and spell resistance that used to help with the impact of a large group are lackingApparently most DMs struggle to build good encounters, or do they?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.