Hope you are doing well.
Downgrading Ubuntu version is possible to some extent. With enough fighting, it can be done.
However the result you end up with is not the same as what you had before the upgrade. During an upgrade certain one-way changes are made to make new packages happy and downgrading them won't reverse those edits. It's impossible to guarantee a downgrade will work at all, let alone if it'll be stable.
In short, your easiest option is always going to be a reinstall. Don't think of it as a chore, look at it as a nice opportunity to trim down on packages and configure things better.
Please let me know if you have any further issues regarding this.
Feel free to contact us in case you have any query.