I'm 5'11" with a thin build of 138lb and a 36" chest. My hips are 30" and waist is 28". I've looked online and my measurements are really weird but I fit into so many categories so I'm wondering what suit size I am.
Unsure of Suit Size
I'd interject a step between 4 & 5 of ensuring you try different brands/ stores as no two companies use the same patterns and so just because you are a 44S in one doesnt mean you'll be that in another and for some brands, none of their sizes will fit you properly.
There are a very few lucky people who can easily buy off the shelf and it fits properly. Most dont care and just get a moderate fit and those that do care either pay for alterations or start looking at other options like MTM or bespoke
In the ideal world, one would get their suits custom made and totally circumvent the problems that emerge from off the shelf suiting. However, the world isn't ideal and money is always scarce!
I'm in a similar situation to you. Although I'm somewhat larger, my sizing is quite odd and I struggle to find off the shelf suits that fit like a glove. When buying suits, blazers or coats I'd personally recommend going to the shop and trying them on. Although there is a clear appeal in online hoping, one can never get a true feel for sizing. Once you have done this (and presuming you've spent enough money to warrant doing so), I'd find a good alterations tailor, take your suit in and get it tweaked accordingly. This way, you can get a fit that works for you without breaking the bank in the process!