The Qlik Sense front end is different to the QlikView front end, as you allude.
The one general thing I would suggest is to work with the Qlik Sense front end and not against it. Attempting to exactly replicate what you have in QlikView into Qlik Sense could well end up painful, time consuming and substandard. If you do exactly replicate what you had in QlikView into Qlik Sense then one could ask the question as to why bother with all the effort to just end up with what you had.
Where possible initially do everything Out of the Box.
That will probably not be achievable and extensions are one of the strengths of Qlik Sense. Create a defined collection of good extensions that meet your needs, but make sure you have the skills needed to create and maintain these extensions either in house or external resources. Open source extensions can be downloaded from Branch / GitHub, but - I repeat - do make sure you have the skills needed to maintain them. They may not exactly meet your needs and you will need to be able to support them over Qlik Sense upgrades especially if any API's get deprecated.
Extensions with support can be purchased, Vizlib | Qlik Sense Extension Library is one vendor, but a cost / benefit analysis would be needed.
I am an advocate of an Agile approach as opposed to Waterfall. Start small and accept that your first migrated app will not be perfect and nobody, especially non techie users, like change.
Q. How many folk singers does it take to change a light bulb ?
A. Twelve. One to change it and eleven to write a song about how much they loved the old one.
Agile allows one to fail fast, fail often and then succeed quickly. Waterfall allows one to fail slowly, fail once and realise you are in a cul de sac. Other viewpoints are available from Waterfall fans.
If you have a local Qlik Dev Group – Eat. Sleep. Qlik. Repeat. then go to it, gossip with those who are migrating or have been there done it. You can pick up a lot with a gossip over some pizza.