Adding more loft insulation questions

Arfa__

I have a fairly new-ish build from 2007, timber frame construction, drywalls and rendered (no brick).  Loft is a 'cold' design, huge amount of ventilation all around with the soffits having open grills. Insulation in the loft is about ~160mm of typical glass wool up to the joists. We have a couple of rooms in the loft, so access is in the eves beside these rooms, with about 100mm of glass wool insulation of the vertical walls, and 10cm of blue rigid foam on the slanted roof sections (with air gap above this). I also have access to above the loft rooms, where again there's similar 16cm glass wool up to the joists. Access isn't too bad, certainly crawling room in the eves and much more space above the loft rooms. Some photos for reference below.

So, first up, not enough insulation, should be 270mm - right?

So, I'm thinking of doing the job myself and rolling out an extra 100mm over a weekend (or two). Any good reason not to do it myself and get some pro's in?

More bog standard glass wool the best plan?

FFP3 mask and gloves job - right? Any other safety concerns?

Aside from ensuring ventilation from the soffit vents isn't hampered, anything I need to be aware of?

There's a lot of pipework in the loft - ok to just cover this? (it's already insulated anyway).

Ditto covering electric cables? (there are no high power shower cables)

There are many GU10 downlights in the ceiling, but all replaced with lower power LED bulbs now, so I assume I don't need to worry about heat etc from covering them some more?

Any particular brands to look out for/avoid? Knauf seems to be the default choice at around £22 a roll.

I reckon I've got about 150m2 to cover (it's a biggish 6-bed house), with each roll covering about 8m2, I reckon I need about 19 rolls, so about £425 outlay.

Anything else I need for the job?

We have a standard gas combi boiler for heating, so million-dollar questions - how much energy do you think we'll save from this job and thus how long before the work pays for itself?

FreeBear

270mm will get you to the minimum standard, no harm in going thicker (I'm going to end up with ~350mm in my loft when I'm done). Covering pipes & cables won't hurt them in general. Just put something over the top of the insulation as a reminder of where cables/pipes are.