My 2 cents ...
I seem to re-call (IS2RC) the advice to upgrade the 1.9 to the highest 1.9.20 *before*
attempting upgrade to 2.2.x.
IS2RC around version 2.3 there appeared an 'upgrade assignments' link in site admin menu and for a while in mod there was the old assignments code + the new assign code.
IS2RC ... themes ... 1.9.highest -> 2.2.highest, there were themes that were never
installed but appeared in the plugins check. Even though it said to be removed
they never were and I had to resort to editing mdl_config_plugins table to remove
rows that referenced those themes. Eventually, one would have to change default theme to whatever came with version ... like it or not. Advise doing that from the get-go and don't worry about the way the site looks until you reach destination version.
There was massive change to moodledata on first hop 1.9.20 -> 2.2.x. One might need to leave on Legacy file system on until that's sorted out. In moodledata a bunch of old course ID numbered directories ... for old certificates issued to students.
Also, once one is successful in upgrading along the march, good idea to update
the environment check to see when one has to upgrade PHP and or MySQL + convert DB to Barracuda, InnoDB, character set utf8mb4 with a collation to match what was in config.php - utf8mb4_unicode_ci. Now if you had been following old directions and copying config.php from old code to new, the DB array never got the line for collation inserted. That would cause some issues/troubles.
Plugins ... some didn't make upgrades to 2 or 3. Check all those for compat versions at each hop of the march. If a plugin no longer exist for destination version it's gotta go - or be prepared to hire programmer.
If 'marching' that far version wise ... strongly advise doing that via command line and using Git for upgrade of core code. But, if you suffer from 'clas' (command line avoidance syndrome) than prepare yourself for some frustration and issues along the march.
IF ... IF ... none of the above sounds 'fun', do what Davo suggest ... it is your path of least resistance/frustration.
End - 2 cents!
'SoS', Ken