Issue with Promise chaining

Hey there,
I'm having some issues with promise chaining and I am wondering if anyone can tell me what am doing wrong.
I have a function that returns a new promise and resolves when an interval function has been cleared.
Something like this:

function func_a(var1, var2, var3, var4) {
return new Promise((resolve, reject)=>{
//math
var pos = 0;
var end_pos = 100;
var id = null;
function draw(){
if (pos == end_pos){
clearInterval(id);
resolve("resolved");
} else {
//drawing function
pos++
}
}
id = setInterval(draw, var4);
});
}

now I have a for loop that is supposed to execute func_a sequentially but no matter how I try with promises it doesn't seem to work.

if (true) {
var p = Promise.resolve();
for (a=0;a<100;a=a+20) {
p=p.then((res)=>{
func_a(a);
});
}
}

for example only draws the thing for the last value of a and

if (true) {
let p = func_a(0);
for (a=20;a<100;a=a+20) {
p=p.then((res)=>{
func_a(a);
});
}
}

draws the first one perfectly followed by the last value of a.

What am I doing wrong?
I checked the references and I couldn't find an async option so I assume that is not a thing for the bangle?

Thanks in advance
ps. I am aware I can do:

let p = func_a(0).then(()=>
func_a(20)).then(()=>func_a(40))

I would just really prefer to do it in a for loop to save a lot of lines of code