Need expert shoe advice

I am fed up of black toenails and nasty blisters on the tip of a toe and need to get this sorted. In the last 2 years I have attempted to purchase 2 new pairs of trainers and both have led to issues with blisters or black toenails and I keep going back to my older Mizuno trainers as they give the least amount of issues. Last gait analysis in January was told that I no longer need a stability shoe (mizuno wave inspire) and i'd be ok with a neutral now. Bought Brooks Ghost 11 as was told they tend to be generous for width. Not the case, I was now getting new blisters on the edge of the small toe. So I am back to the old mizuno again. But even they are hurting after 4/5k and it's really getting me down. Left foot is slightly bigger than right so I understand that is why it's the more painful. Right foot has a black toenail (a nail which has previously fallen off twice now I think) Left foot 2nd toe lost the toenail off it a few months back and now am getting very sore blisters, sometimes blood blisters, on the tip of that toe. A small nail is present but not fully grown and maybe not quite full strength yet. When that toe hurts it feels like the toe nail is being ripped off with every step. No other toes get affected much, the odd wee blister but they dry up quickly.

I am a size 6.5-7, with the left foot being nearer a 7. When I was sold the brooks in January I was given a size 7. The old mizunos were a 7. I have now gone and bought some more mizunos but a neutral shoe and got a size 7. I've not worn them out yet, only round the house, they feel ok, but I am so paranoid that once I get running and the feet swell a bit that the same will happen again (and I can't afford to lose money on another pair of trainers). I tried walking in them without their insole last night and what a difference I feel in the toe bed. However they have stitching right round the inside so that would likely rub if I had no insole.

Now for my question!!

Is it wise to swap in an ultra thin insole to allow me to have more room, will this affect the comfort or performance of them? If this is ok, can anyone recommend an ultra thin insole?
or
Should I be looking at swapping them for a size 7.5 and just hope that is the extra space I need?

I also mentioned above about the 2 pairs of trainers I bought in the last 2 years, the others were saucony guide, same shop, sold me a size 8 in these and I started getting blisters in areas i'd never had before so decided they'd fitted me to big that time, can't actually remember if my toes/nails were ok, but I stopped using them after a few months so probably not. Why do I never get fitted with a 7.5, seems the obvious size for them to try on me but they always seem to be happy with the fit of the 7 so I trust them,

Sorry for the long post, hoping there are some proper shoe experts on here that can give me some invaluable advice.

p.s. - I already use feetures thin running socks!