There are so many vowels to say in 16th note strumming, you can only do it very slowly. Initially you need to do it this way, but the goal is not to memorize the count, the goal is to memorize the groove, if that makes sense.
Guitar Pro is what I use. I mostly use it for getting the timing right for riffs and you could do rhythm parts as well. I’m sure I remember Richard did a club session where he put old faithful into GP and you could do the same for16th patterns.
When i come across pieces that require 16th note strumming, i find it helpfull to draw arrows (up and down) above the note.
Also, tapping my foot helps a lot.
But as with anything guitar related, start slow. These are things you have to build up really slow. With practice you’ll notice that: a) you get the rythm solid in your head and b) your hands (and your foot) will follow.
Next you’ll find that speeding things up will be easy.
If anything, try clapping your hands to the beat before playing the rythm on your guitar.
Tony, you need to do many, many repetitions so the sound of the rhythm enters your ears and head and becomes recognisable as a rhythm, not as an abstract in written form.
Muted strings, slow and steady is the way to go.
That helps less because you are starting from the position of not hearing the overall movement of the offset, the asymmetrical sound, that such a 16th pattern creates.
If seeing as two bars of 8ths I would still encourage playing from the very beginning as two consecutive bars, not two separate pieces.
You should work hard at tapping your foot to the beat. As below. If your foot starts to tap the strumming pattern instead of the beat, then this is a problem you need to fix. Listen to the piece and just tap your foot to the beat. Do it for a while to really lock in. Then start the strumming pattern and keep your foot locked onto the beat. It can be difficult to start. Being able to lock onto the beat with foot tapping (or head nodding or body swaying) is very important!
Make sure when you practice you mute teh chord and focus fully on the rhythm.
I would try counting it out aloud too - verbalizing can hekp get the pattern into your musical imagination!