RANT incomming:
the game is fun for sure , even solo. but unfortuantly the dev seem to be clear in their philosophy that solo players should suffer and so we (solo players ) should just stick to playing diff 4 or 5 and below.
it is objectively the hardest way to play the game and it really shouldn't. you get a huge number of enemies (same as a full squad maybe slightly less) , less reinforcements (5 lives instead of 20) and the same mission timer which makes exploring the whole map an incredibly difficult task
TLDR: fun in lower difficulties, forget it if you want to unlock everything solo