Migration strategy from ownCloud

Hello, I am new to this forum because thinking about migrating from ownCloud to Nextcloud and moving to a new server simultaneously. My ownCloud installation is at 10.0.10 on a Debian 8.11 server with Apache 2.4.10, PHP 5.6.40 and MySQL 5.5.62. Target system is a Debian 9.11 with Apache 2.4.25, PHP 7.3.14 and a yet to be defined MySQL version. Currently disk space consumption is about 60GB in data, and is planned to grow to approx. 100GB

What I learned from the docs here, it seems that I have to migrate from ownCloud 10.0.10 to Nextcloud 12.0.12 first, staying on the same server. Then I would have to upgrade Nextcloud to a more recent version (which?) on this server and only afterwards copy the whole installation to the new server, possibly continuing the upgrading there until Nextcloud 18.

If that would be a feasable way, what would be the Nextcloud version to upgrade to from 12.0.12 on the old server, since I have read that Nextcloud 12 is not running with PHP 7.3?

Another question is, reading the docs and the advice to delete all ownCloud stuff except for data and config directories, I think the same result could be achieved by copying only the data and config dirs to a new location (and vhost) and doing the migration there, leaving the old ownCloud installation untouched (to be on a safe side). Is that right? And BTW I have moved the data directory out of the ownCloud installation directory (but on the same server) - would that be a problem?

Last, what about client programs? Do my users have switch from the ownCloud clients on their various devices at a certain point during the migration or are ownCloud clients supposed to be working with Nextcloud server and/or vice versa?