After going through endless variations in position, generally to accommodate injuries, I always end up exactly back where I started, 35 years ago, as far as position goes. This is essentially a 1970s classic road racing position.
Sliding forward often indicates a saddle too far forward, disturbing balance.
Sliding forward often indicates too great a reach.
Too great a reach often results from an inappropriately sized frame or too long a stem.
Discomfort or instability on the saddle often results from inappropriate drop.
There's lots of complexities possible. Most people seem to do very well with a bike sized for their height, top tube/stem length appropriate for that height, cleats a bit further back than modern fashion to move the foot more over the pedal (less calve work, more in the upper legs & butt), and saddle back until weight on the handlebars is right. Also, handlebar drop appropriate to the size of the frame & rider.
I ride a frame slightly larger than my ideal because the frames go from 54 to 56 and I could use a 55! I chose one size larger because I'm more a distance etc rider than a racing type guy.
The best simple setup/sizing system I've seen is at Moulton's site. He's forgotten more about making bikes work than most people ever knew.
prodigalchild.netBicycle6.htmOpen ↗ works very simply. I end up with a 55 x 55 as the ideal. I could go with a 54, but they're so short and tight that I choose a 56 x 56, with a slightly shorter stem. Also makes the bike handle more like I'm used to. Of course, a Colnago (slightly short top tubes) would be ideal, but people seem to want $$$$ for them!
So. See if the bike is the right size. If you're one size up, which is usually the case, you can probably get a shorter stem and you'll be fine.
And get the drop right for a road bike. See davesbikeblog.blogspot.comhandlebar drop.htmlOpen ↗ I work up and down, always end up at 9mm drop in a distance bike. Which no surprise is what Moulton's chart shows.
OK. So the reach and cleats and height right, then back the seat up and lower height gradually (or the other way) until bingo, smooth power and spin, comfort, etc. The magic spot is quite nice and generally further back than today's racers look like they're using.
Then your butt is in the position that works and you find a saddle that supports it really well there. Different range and use can call for different saddles. For short rides (40 miles & under) I love a narrow, really secure saddle that locks me into a tight zone of supported comfort. Usually that type of saddle turns into an a** hatchet after 50 miles, at least for me. So I tend to ride a heavier, clunkier saddle with a wider nose and longer sitzbone support zone. I can shift around a bit, ride on the nose a little, slide back, and generally keep discomfort to a minimum.
Problem. Good saddles that I like seem to cost lots of money. The short range saddle that works super for me is the Fizik Pave, old model. Really free to spin up etc, almost not there. But the rounded profile of the nose that gives this freedom tears thing up after 50 miles. The relatively tight sitzbone pocket that provides stability on short, fast rides gets old. The suspension feels a bit stiff after 3 hours, too.
So. I use a different Fizik with a flat topped nose and longer support zone. The Pave is off the bike now that the season is in swing and I'm doing 200 miles a week. Both my Fiziks are obsolete and were pretty cheap on eBay.
I'd get fizik.comcatalog.aspxOpen ↗ if I were shopping now and made lots of money.
The cheap seats don't seem to work as well, although I've used Specialized with some success. roadbikereview.com has lots of opinions.
Anyway, get the fit right and then get the saddle that works with your fit. Don't put the saddle too far forward or everything will hurt!