The more I think about this, the more it should have been Avengers: Secret Invasion (or Captain America: Secret Invasion, a la Civil War) to cap off phase 4. I know they wanted to give Avengers a rest, but it'd be a different type of stakes and with a little bit of seeding (like the post-credits in Far From Home and WandaVision) might be the natural culmination of phase 4.
You can basically follow the Civil War format:
Captain America (Sam Wilson), Ant-Man, Wasp, and War Machine are on a mission, tracking down some D-lister, or the head of some criminal organization. The bad guy dies in the process and is revealed to be a Skrull.
<Marvel Studios intro>
Rhodey contacts Captain Marvel. They begin to suspect that a rogue sect of Skrulls have infiltrated Earth for some nefarious purpose. Wasp suggests that they go to her father for help developing some kind of Skrull tracker. Hank Pym has been replaced by a Skrull. At some point you can have some kind of "someone in this room is a Skrull" paranoia, a la the Thing, and Ant-Man is revealed to be an imposter as well. You don't know who to trust.
Eventually they decide to recruit potential heroes that the Skrulls wouldn't know to replace. For example, Shang-Chi is an unknown, but Captain Marvel has met him. Ms. Marvel is a new hero that hasn't met anyone. Maybe there's a way to get Kate Bishop or She-Hulk involved. Maybe they go to Wakanda for Shuri's help but she's MIA. One of the aides we saw in Black Panther 2 was a skrull all along, and now M'Baku has been replaced. They may be trying to start a war between the U.S. and Wakanda so that skrulls can inherit the earth.
Meanwhile, Sam Wilson contacts Bucky (who is pretty off-the-grid and couldn't easily be captured by Skrulls), Bucky gets with Val, and they decide to implement the Thunderbolts program by recruiting former villains who have been imprisoned, because why would Skrulls replace someone in a maximum security prison? So this team would be Ghost, Zemo, Abomination, and Florence Pugh's Black Widow (another unknown that couldn't easily be captured).
This all culminates in an Act 2 battle at a German airport, where Skrull versions of the new Avengers (Cap, Wasp, Captain Marvel, War Machine, Shang-Chi, Ms. Marvel) fight the Thunderbolts (Bucky, Val, Zemo, Ghost, Abomination, Black Widow) until the actual version of the new Avengers shows up as well. Who is who, who is a skrull? Lean into that. Maybe Shang-Chi can easily chase down faux-Shang-Chi because the fake one doesn't know kung fu.
Anyway, finale takes place on the white house lawn, skrull imposter team vs. hero team, maybe Skrull President tries to order the nuclear strike, real President tries to stop it, maybe even good-imposter Talos President adds to the paranoia, what's going to happen, etc. And in the end Val kills the skrull leader in front of the whole world, launching her (and whatever nefarious agenda she may have) into prominence for Phase 5.
I don't know, I came up with that in 5 minutes. But you could do all that with a $212 million budget, and actually bring together Avengers new and old, and have a phase that actually means something with various Skrull teases and imposters throughout, rather than... whatever this is that we have now, which just drags things out, damages the brand, introduces characters who still haven't been seen again... etc. They need to start tying this **** together now before everyone jumps ship, including this massive MCU fan.