QGIS 3.10 problem with merging layers & multipart promotion

Good afternoon,

I would like to merge a few layers in QGIS. However, I can't do this. When I run the MMGIS option, then I am getting:

Merged input_layers must all be same type of geometry (MultiLineString != Polygon)

According to the MMQGIS author, all layers must be in the same geometry.

In this case, I set all the same geometries for my objects:

But it didn't solve my problem:

So I tried another way:

In the Vector - Data Management Tools - Merge Vector Layers I selected my layers. Unfortunately, I got sth like this:

According to this question:

Merge layers with different geometry

I used "Join attributes by location" option, although afterward, I got only 1 layer, even when I select "Create the separate features for joint..."

Should I use the GRASS as per below?

Error in merging two shapefiles of different geometry types

I would like to have all the layers in the picture below merged:

Or should I maybe do sth like this:

Merging layers into one- QGIS

UPDATE:

As per the advice below I tried the "Promote to the multipart" option.~ Unfortunately, it doesn't solve my query too, since this tool can promote only 1 layer:

In the effect I have only 1 layer "multipart", but how about other ones?