I think Siege is a good choice.
I play there as a newish player, and enjoy it, always plenty to do, and achieving little things are actually an achievement - like getting some bandages.
I have everything on production, and often find it hard to be motivated or do anything - but, there is a feel you can play wider content more easily - with 7 characters, more skills, and therefore more content available. But, I often look at my fully developed account, with every template minus a sampire, and think, I cannot actually do any of the game content without a sampire (and I hate sampires so much), that's how it feels. {Although the Devs seem to have been addressing this a bit, and bringing in more content for other playstyles}.
I do enjoy the crafting, but the crafting results have no meaning on production compared to too high end pvm loot which has flooded the market.
Starting from scratch, is in fact an interesting and very fun experience - and it can still be done on Siege - I'm currently doing it myself after being 20 years on production (less time in WoW).
Yes it's hard work, but you are looking to farm millions of gold, which is still possible, rather than platinums on production shards, which is not possible.
I know a lot of players feel they have a lot invested, and feel too tied to make any sort of move.
You can buy yourself into the game on all shards, Production and Siege, but I would not recommend it, you lose the feel of game achievement fast.
Atlantic sure is a crowded shard, there is a lot of life there. Other Production shards do have life also. Europa etc. Some are dead quiet, you may want this.
Some pro's and con's of shards;
Atlantic.
The most casual pvp.
The most crowded/active.
The central marketplace.
The most screwed up economy.
Almost need to buy your way in.
Can train characters and templates up fast, get a wider feel of the content.
A more new school UO. Highly itemised.
Siege.
The most sense of community.
The best run economy.
Less pvp, but could be the best pvp shard if it got the players.
A good marketplace.
Can start from scratch, and get the real feel of the game.
Will need a lot of soulstones, to handle the 1 character thing, and takes a long time to train up under ROT. Lot of players just don't have the patience these days.
Closer to the original version of UO.
There you go, a few of my views.
And even though it may look like it, I'm not promoting one over another, I enjoy production on Europa, and I enjoy Siege, that was me being balanced.
And welcome back by the way, hope you enjoy.