Congrats on the new physique! I'm always trying new things, so I can't say that I'd use all the same clothes....
Sure, check the thrift shops. I've found a few great pieces there, but my experience is you'll spent a lot hours to find something (and now with a reduced lifespan.) I love buying second hand and if you do too, check the gear swap/used postings at your local outfitter, craigslist, and the bpl gearswap. Many times you're getting top quality kit at (or close to) the price of "big box" garments.
I didn't see in your original post that you were budget focused, but I second many of the opinions that you can outfit yourself on the new, for less than retail, if you want to. I am often willing to spend more for more comfort and quality.
I am a big fan of getting stuff to fit perfectly, so if I was in your shoes, I'd be trying out different types of garments from many different manufacturers. Not just springing for some cheapie track pants that will be rubbing together every step you take...unless that is your desired fit! I have had some champion branded poly baselayers that fit and performed well. (target?)
For baselayers and pants: I've had Patagonia pieces for most of my outdoor venturing years and keep returning for the long lasting quality, features and fit. (And they do all the green stuff so you can feel better about consuming...) I like the rock craft pants for 3 season use, they fit my skinny "hipster?" legs well. (No zip off legs for me, just roll 'em up) I have no love for their h2no shells. I have some older guide pants that still work well for winter conditions. The patagonia merino 3 hoody that I just picked up off of gearswap is my favorite new piece.
For shells: I like Arcteryx for the same reasons, their gear seems to last and hold value. (Once upon a time I wore a softshell for 4 years, sold it on CL for half the original retail.) I seem to get twice the life out of their garments as stuff half the price...hmm.
Others I've liked: OR shells in paclite, merino wool baselayers/midlayers (ibex, patagonia, the redram/icebreaker stuff on STP)
Not what you asked, but walk into your local outfitter and pose them the same questions. Similar to the internet, you'll get a bunch of opinions, but you'll be able to see stuff IRL and gauge quality and fit, and bonus keeping your money in your community. If you lack any good local options, pick one of the many online retailers with competitive pricing and easy returns and start trying stuff out. Keep in mind your pre-svelte gear experiences!