I think a lot of what’s been written so far is talking about what we see at the pro level. I don’t know if that’s the right reference point.
This. And at "shitty amateur" level, physique doesn't particularly matter; and what does can be changed in the gym (or McDonalds if looking at front row).
That's why I'm suggesting to pick a position based on what you enjoy doing; and to do so whilst being guided by your local club coach.
When I suggested getting a few matches off the bench first - I'm thinking of shitty amateur level, where the bench has 0-3 people, who fill in as required. If forwards, chances are any bench man will play flanker - where they're least likely to hurt themselves, any back is likely to play winger where they're away from making any decisions.
Bit late to the party but don't overthink it too much dude! Go down to your local club and they'll find you your level. If you want to play 8 go for it but a coach could easily put you 6 or 7. Don't get discouraged, put your body on the line, be sound and everyone will welcome you.
We're all playing for fun so effort and attitude are far more valuable traits than being the most skilled or most athletic guy on the team!
And this too.
Ultimately, you're 21 and returning to rugby for having had a few years out. When you did play, you played prop.
You're not turning professional any time soon; with the possible exception that you're already a semi-professional sprinter but didn't tell us that.
Starting off the bench at shitty amateur is your level right now. At shitty amateur level, physique doesn't really matter a damn. I'm 182cm, and I was the shorter of my team's 2 SHs; I was also taller than one of locks.
Skill wise, my sprint was barely quicker than my fast-jog, but I could fast-jog for 2 hours without a break, and I had a good pass off either hand, my competition at SH could box-kick and had a better R=>L pass than me, but his L=>R was more even laughable than my box. He was faster than me, but his lungs were busted after 5 minutes of phase play (not that we were good enough to hit 5 minutes of phases without someone dropping the ball of opening the gates in defence). I typically covered FH in the event of injury because no-one else could get their head around making a decision before having the ball in their hands (or a pass that could approach ours, hell our IC couldn't even spin the ball off his R=>L).
That's the skill requirement at shitty amateur. You need to be able to play on instinct to play half-back; you need stupidly strong core to play front row; beyond that, nothing really matters (though height is useful at lock).
You've every chance of evolving into a shitty amateur starter, or even in a few years, reaching reasonably decent amateur starter - by which time, you'll have played pretty much every position at some point in time, and not be needing our advice.
Seeing the conversation between SA... and NZ... It's often claimed that the reason England has so many more players than anywhere else is that our depth pyramid goes deeper into the amateur level and shitty levels. I'd always thought it was just a cop out, especially compared to the likes of France and SA who also have tonnes of players; and Wales and NZ where rugby is what everyone does. Maybe there's some truth in it after all.