Can't create more than 4 primary partitions.

[deleted] Reinstalling W10 would mean that I will lose all programs-this is just too much work-I prefer to boot from W10 for my father and when I need to use my PC I will boot via F12...
Although are you sure both of them must be only MBR or GPT?
On multiple places there is that list of commands that you mount different bootloaders, then do some commands, from which the final one is update-grub(your favorite xD).
I may be completely wrong, but it sounds logical to me to show the locations of different bootloaders to GRUB manually and tell it to give you choice when you boot(like Solus-dev/sda....; W10-dev/sdb...; if choose bootloader 1-start Solus, if choose bootloader 2-start W10).
https://askubuntu.com/questions/681422/grub-menu-not-showing-with-dual-boot-uefi-mode-installation

I just downloaded waterfox, but I have to remember where I put it and I can run it by opening folders and clicking on the icon-is there a way to put waterfox where other apps are and create shortcut-an icon to open it just like regular apps.
P.S. I used the script provided in Joplin website and it's working just fine It automatically installed the app and it's looking just like the others. That's what I want to do with Waterfox-I need separate browsers-one for accounts and the other for browsing.
Waterfox was downloaded in temporary files, which I think would be deleted so I moved it into downloaded.
I am confused by the file manager for the root and home partition-is there a folder just like in W10 like program files-and that is where the apps are stored with their settings, libraries and so on?
Don't know how Joplin and waterfox will update though. Waterfox has an icon for updating, but I suppose it will run it automatically by itself.

Well guys we've got to your favorite part-drivers
The problem is in my speakers-there seems to be plugged a device and all the volume settings are at 100% but there is no sound. That being said this speakers were problematic even in W10-I install their drivers and after that I had to go into some settings to specify something about that they are connected to the front ports of the PC...although that seems common for Windows.
I opened the drivers program in Solus and it said nothing is found-so the problem won't be resolved so easily.

algent Given the complexity(different HDDs,different formats of HDDs,different formats of bootloaders) I won't be able to do it myself-for now I use F12, but it's buggy and sometimes I got to choose between something like solus 4.xxx.xxxxxxx and solus 5.xxx.xx -I go with the 5-it's the default where the PC boots automatically after 3 seconds.
There are 3 additional problems-speakers won't work and I can't find the directory of Thunderbird's calendars nor can I import them...I haven't found another locally stored good calendar except Thunderbird.
And the last one is that I downloaded and unzipped waterfox, but I store it in downloaded and I have perform like 20 clicks before open it-I can't put it where other programs are(I don't have permission-no option even for typing password)-well I don't know what I am doing so perhaps I wouldn't be OK even after I move waterfox xD When it's opened I don't have an option to make it favorite.