Introduction and color questions

Hi everyone, I'm new here :)

Anyways, I have a long set of questions about hair dye.

My natural color is basically a dark blonde with some red in it. It might or might not be called strawberry blonde, it's sort of a golden color.

I've been trying to get my hair to the color I want for some time now, which is a medium or possibly dark (but not too dark) brown color. I also want it to be quite ash, and not auburn or have any brassiness to it.

I also have my hair permanently straightened, so I want to try to do this in a gentle, but affordable way.

The last time I dyed my hair "medium ash brown" and it looked great at first. Basically it turned a very chocolatey brown color, and dark, but not quite dark enough to be mistaken for black. It's been quite a few months ago now though, and it's all but faded completely

I want to get that color back, but I want to keep it the color it turned out to be right after it was dyed. About 6 weeks after I had dyed it, it turned to sort of an auburn color, and it's been getting lighter ever since.

I try to follow the rules to keep my hair color longer, like not going in the sun too much, not going in swimming pools, not shampooing every day, and using a color safe shampoo and conditioner (I'm still trying to find a really good one here, all the ones I've used still seem to strip color). Someone once suggested brunette expressions by Pantene to me, but the regular color safe Pantene I tried really dried out my hair, and I'm worried the brunette expressions might do the same thing.

Another thing I've thought of trying is going a little darker to try to preserve the color. Dark ash brown is definetly too dark though, it looks almost black on me, which makes me look goth or something. I wonder if a "4.5" (medium ash brown and dark ash brown mixed) would be ok though.

The other thing I've thought about is asking a stylist if I can use a temporary (semi or demi permanent) color at home between dyes to keep my color looking like it did right out of the salon. My stylist who just restraightened my hair actually suggested that I skip going to salons for permanent dye and just use temporary hair color at home (partly because it's less damaging). I'm worried about several things if I go that route though, ie: different companies have slightly different hair colors from their dyes, I don't want to get the dye in my eyes, I'm worried about leaving the dye on too short or too long and not getting the right color, and I'm worried about getting the dye into the hair evenly.

Can anyone advise what to do? Should I only use stylists to dye my hair, and use lots of permanent color? Should I use permanent color (from a stylist every so often) and use a semi/demi-permanent color inbetween, or should I go entirely to semi/demi-permanent colors at home, and if so, what should I watch out for, how hard is it to do, and what should I watch out for?

Thanks in advance!