I'm starting back out again after 10+ years of giving up stretching.
Back then I could easily take my fist up my ass with only my spit for lubrication and could take some really big insertions and my ass-lips, after being worked and swollen, protruded beyond my butt-cheeks.
But In the intervening years I've returned to having a normal 'micro-arse' - and a few weeks ago, starting back again, struggled to even get a finger up!
Since then I've been making steady progress, with a short session (half an hour) every two or three days.
My advice is maybe a bit left-field - but might help: it is that heat makes things a lot easier.
Heat relaxes your muscles, it feels nice inside you and helps you be more aware of what, and how much, is inside you and where it is - all of which I find helps with comfortable insertion.
When I started at zero a few weeks ago I'd been struggling with a small butt-plug - just 4cm in diameter - it just didn't want to go up.
Then I remembered I had a small crystal ball - 4cm in diameter, like the butt-plug. I warmed it up in hot tap water for a quarter of an hour till it was hot, but not too hot (your asshole will tell you, at the slightest touch, if something's too hot!).
When I tried putting that up my arse it slid up really easily and felt so good, the sensation of width and heat travelling up past my ring and into my rectum.
I've found that with all sizes of insertions - it's easier if it's very warm or hot - you feel it better, and once it's up it gives me a sensation of euphoria!
The trouble is that it is (as far as I can tell) impossible to warm up silicone toys to a decent temperature.
But there are things that can be be warmed up and safely shoved up - potatoes work fine.
Myself, I like to fill jam jars with hot water - I only use ones that are thick-glassed and that contained vacuum sealed, cooked foodstuffs because these are made with very tough glass and designed to be highly resistant to pressure.
But I recommend this with great caution - I've been shoving for decades and never had a problem - but, as I'm sure anyone can imagine, using jam jars is not theoretically risk free. But there's nothing that feels so good as a big hot water-filled jam jar sliding up past your ring and nestling, warm and comforting in your rectum.