1. What would be the best way to move many databases from older XAMPP installation into newest one?
If the release step from the previous installation to the new installation is not too big (there is no other way than simply to try it), overtake the whole "data" folder from c:/xampp/mysql (i.e. c:/xampp/mysql/data). That means, backup your old Xampp, deinstall it, install new Xampp and finally rename the new data folder to something like data-backup and then copy the whole old data folder to the new Xampp release. Then start Xampp and "pray" for it...
If you are lucky, you are already done.
If not, you may use the command line tools instead of phpmyadmin, which are provided by Xampp. There is a tool called "mysqldump" in the bin folder, this tool can export tables, databases and also the whole SQL environment (all databases). Enter "mysqldump --help" for an explanation of all valid options. Mysqldump will create one large text file with CREATE and INSERT Statements for all data. After creation, you may import that file via "mysql" command line tool or also via phpmyadmin, but keep in mind, that it may exceed the maximum upload size of your php.ini configuration, in that case you must edit php.ini and assign a bigger size to upload_max_filesize, then restart Apache. In some cases, you also have to increment the value of max_post_size as well. Last not least PHP may run out of time, increase the value of max_execution_time.
2. is php7 xampp stable enough to be used without problems?
I dont know.
3. Does anyone have XAMPP installed on SSD?
Yes, i have replaced the HDD in all my laptops and desktops by SSD. Due to the price drop you already mentioned. Even if does not solve your Xampp problem (i dont know if it does), it generally speeds up access time dramatically, it speeds up booting as well as starting programs. Its worth the money!
4. How to speed up XAMPP on WIndows 8 and 10?
As you are using big amount of SQL Data, you should try a different configuration for MySQL (or MariaDB, this is a binary replacement for MySQL). In c:/xampp/mysql/bin/my.ini the MySQL Server is configured. This default file is ment for small amount of data. One step higher, in c:/xampp/mysql you will find some different configuration files, these are called like "my-large.ini" and "my-huge.ini" (and also "my-medium.ini" and "my-small.ini"). The default file my.ini from the bin-folder is a copy of "my-small.ini". In your case, try to use "my-large.ini" or even "my-huge.ini" (simply copy to the bin-folder and rename to my.ini, of course remove the existing my.ini before). Then restart MySQL. I think there is a comment in the ini-files, when to use which configuration, but you may simply try them. I dont know, if this speeds up your Xampp, but it is the only feature i know for general MySQL purpose.
It is well known, that there are performance problems with Joomla, this is not a question of Xampp or Windows, it is a wide spreaded Joomla problem. There is no unique reason, there are many problems with Joomla, for example large data amount, too many plugins installed, very deep nested function calls and and and. The internet is full of Joomla problems. I dont know any general advice how to speed up Xampp. You have to speed up Joomla, it is a Joomla issue, not a Xampp issue.
P.S.: Anyway, if you have success or not, i definately recommend to use Linux instead of Windows. Many problems (as well speed problems) disappear automatically under Linux. But instead of using Virtualization (this is handy for a test of another Linux distro or similar), i recommend a "normal" installation, simply insert a new SSD additionally and install Linux to that SSD. Linux will install a boot manager (grub) which gives you the choice of starting Linux or the already existing Windows Installation. When it comes to speed, a normal installation is far superior to a Virtualization.