Chartered engineer in UK with just under 7 years experience probably about to make the move from building design to forensics. Spoke to my old boss who reached out to set up the interview for me. He was pretty candid about what he was being paid when I asked him if he minded sharing. Sounded like comfortably more than I'd expect to be making in design and he's been there like 3 years and had solid raises each year and a big bump when joining. Sounded like they have very low turnover as well which is a good sign. This is probably somewhat specific as well, but they do no marketing and more jobs appear by word of mouth than they have the resources to do, which is a far cry from my current design led firm which goes incredibly hard on networking and marketing.
Bridges are probably harder to have your own business in and unless you have amazing contacts I suspect starting a forensic consultancy which only does bridges will be incredibly hard because it'll be very niche. The vast knowledge and experience rule will apply even more so to forensics work.