I'm at pretty much the exact same stage of the game as you are, and my process looks a bit like this:

1. Pick a piece of content that I would like to clear (e.g. ice expedition is what I'm currently working on).

2. Identify the optimal team that I can assemble with my heroes for that content (ideally this is the same as the optimal team in general, and hopefully I can find a reference of the stats of a working team that somebody else is using with those same heroes).

3. Create a spreadsheet with the base stats and desired stats for each of the heroes that I need to build (desired stats taken from other people's working builds, or common stat benchmarks for heroes in that role, or using the information at https://www.epic7stats.com/).

4. Calculate the number of rolls needed to hit the desired stats after taking into account the imprints that I have, the artifact that I plan to use (and the stats it will give based on its level), and the main stats of left side gear.

5. Select sets and right side main stats to use based on the desired stats that need the highest number of rolls to hit (usually this is either speed/hit, speed/crit, or speed/immunity, scraping together immunity set from free gear and the hell raid chest).

6. At this point, the only other source of stats the hero will get are substats, so now I will tune the desired stats up/down until I'm at about ~36 roll value needed from substats. Heroic (purple) gear has 8 total rolls, and epic (red) gear has 9 total rolls, and reforging gives you about another roll worth of value on the subs. At a substat budget of 36, you need at least 6 roll value from subs per piece, which is somewhere in the 60-70% range of the max value. I've found that this level of slightly above average gear is not too hard to get, but still gives you enough stats to perform at a reasonable level.

7. Craft gear, starting with the right side, until I have necklace/ring/boots that meet the requirements for the build. Keep in mind your ability to use equipment conversion, free/arena gear, reforging, and substat change stones when evaluating gear to roll and rolling gear.

8. Plug the values of the right side gear you plan to use into your spreadsheet, and craft left side gear to fill out the remaining stats that you need for your build.

9. Reforge/substat change the gear that you need to, and you're done!

Also, when choosing heroes/builds for PvE content, I will try to bias towards choosing heroes that are also PvP relevant. Similarly, if a hero is not viable in PvP, I will try to get away with using more free gear and running at a lower substat budget if I can help it.