When should i play DLCs ?

Hi, im pretty sure im gonna buy all the DLC during the sale, but i heard they make some big change.

Basically, i thought it would be best to activate them when the base game would start to run out of steam or when i saw most of the content.

BUT i have no idea about that so i'd like more experienced people to tell me about it please🙏.

I played around 22h, completed the game or obliterated on monsoon with around 4 character i think. And i also tried to do some unlocks i have all character but 2.

Thats my current progression.

Also what about eclipse thing ? I like playing mercenary and would like to bump the difficulty with it, but i saw no options. Do i have to beat the game on monsoon with every character ? (i never tried the one with little to no mobility as i feel like it would be less fun to play ^^').

And what about artifacts ? i thought i would unlocks them by beating the game but still have none..(i know a quick search on the internet would tell me, but i'd like to spoil myself as little as possible).

Thanks.