Moving away from ratings for a moment, when they do start shooting again Melissa Benoist is going to be super pregnant, so how's that going to work if the start of the season is concluding the story from this season.
Given how badly they've failed to hide Danielle Panabaker's pregnancy I don't hold out much hope.
Hasn't David Harewood said in March following the production shut down, that himself and Melissa Benoist had finished shooting their scenes for the 5x20? If it is true, that could mean their presence will not be necessary once the filming will have resumed (and besides, Benoist will still be in her maternity leave time, which is 12 weeks with certain conditions, in USA, right?!). On the other hand, according to interviews given by Nicole Maines (she has said that her scenes included Staz Nair and Julia Gonzalo, what could mean they could be at Catco), Jon Cryer and lately Chyler Leigh, we learn that they still had some scenes to shoot so when the confinement will be totally or partially lifted at Vancouver, they could return ending their shooting in taking into account the new post-confinement (no physical proximity between the actors, what means no kissing, no hugs, nor face to face unless actors are at a certain distance from each other, no crowd scene, etc... .).
-> let me tell you that for some time, shippers (Alex/Kelly ; Kara/William - if writers keep insisting about their supposed romance -; Nia/Brainy - even if now, they don't talk togather a lot - ; J'onn/ M'Gann and of course Kara/Lena whose their hugs were famous from s2 to s4!) risk to be very frustrated for the lack!
About the 5x20, once finished and finally edited, the episode will surely serve as premiere for season 6.
And, I don't see the shooting with Supergirl cast entirely reunited resume before the end of the year see early next year... when it is the coldest in Vancouver!
Chyler Leigh's interviews to TV Guide
https://www.tvguide.com/amp/news/supergirl-chyler-leigh-season-5-finale-alex-new-persona/)