Glad to hear you want to have a go at freediving!
Certainly it will be beneficial to practice some dry breathholds - but I think it's also useful to have a bit of instruction to help you while doing this.
For example: how to prepare for the breathhold (i.e. how you can learn to relax well, and what muscles you use to breathe beforehand); how to take a full final breath (though it can also be useful to do some half or even 'empty' lungs breathholds); how to do recovery breaths afterwards, etc.
Concerning the mask, I'm curious why you want to wear it during dry breathhold? -Is it just to get used to the feel of it on your face?
TBH, though, I'd say that probably the most important thing to learn & practice for freediving will be equalisation, if you've never done that before...
Many people who have never actually tried freediving think it's all about breathhold, and that will be the most limiting (and scary!) factor. However, if you're trying to dive deeper (e.g. down a line, etc.), then in practice it turns out most people find they run out of equalisation before running out of breath.
If you think about it, getting down to 20m (and back!) will typically take not much more than about 45 seconds - and almost anyone, after a few minutes of instruction how to relax and how to take a good deep final breath, can hold their breath that long, even when (gently) moving.
So, if you've not had any breathhold training/instruction before, and you find you can already manage a breathhold of something like 1m30s (which most can after a couple of practice holds), my recommendation would be that you don't worry quite so much about that aspect, and you concentrate more on learning to equalise if you've not done that before (in particular, using the Frenzel method).
Hope that's helpful!
