Seeing this is your first post, let me assume you are new to the game and say, Welcome!
But since I cannot fully satisfy the request, let my next suggest to make backups and be willing to make dummy leagues to test if things pan out as you would want.
I will post a new variation on the 38-game schedule that gives home/away matchups against each other team in the regular season. But though you might see
claims of being able to schedule spring training and playoff games in custom schedule files, I do not believe that is the case. I would be glad to be explained how this is wrong, but in my experience specifying spring training games either do not work at all or mangle the regular season schedule, and pre-defining postseason games squashes the playoffs as determined by the season.
Be aware that the game may not well handle situations with the league "season" crossing the end of the year on either side. I picked late Feb as the regular season start, which allows plenty of time before for a 5-week spring training, and allows for some weeks after the season for playoffs. That regular season begin time can be edited in the schedule file.
For spring training I believe there is a reasonable way around it if using the file. Set the League Settings > Options > Spring Training Options to have a 5-week spring training. The game will automatically schedule a full 5-week spring training at the time it is to start. You can then go through League Settings > Functions > Edit League Schedule and individually delete all the games except for those on Saturdays.
For playoffs however, I do not think there is a way to have it work exactly as you want. It may be worth asking about this in the main forum.
Under League Settings > Options > Playoff Settings (far bottom right) there is the ability to set some things, including whether "Break Ties with tiebreaker games". The Customize Playoffs button allow to pick the type of playoffs, but the game limits to those structure options. You are not able to say completely open-endedly, "here are the playoff game slots, and I will put the teams in."
What you describe for games sounds like the
AFL final eight system, but that is not one of the options available. One of the double elimination choices is probably the closest you can get. Or a top 6 (winner plus 5 wild cards) with the top two teams getting byes could be more like what you want. You are able to edit the teams in the prescribed matchups after they are set, which can be useful like for fixing which team to have home field advantage. But you do not have complete control.
The other big issue is when the games are. The game will schedule playoff series to begin will one offday after the end of the regular season or the previous round. It might be possible to work around that manually, but I cannot guarantee that. And it might work but only if you do it very precisely and make the change at certain times depending on the playoffs structure. Beware these edits may break the playoffs and end them prematurely
What might work is again to use the Schedule Editor option like to delete spring training games but to schedule playoff games. Schedule (initially additional) games with the matchups on the days you want, like first Saturday after regular season end, then go back and delete the original automatically scheduled games, like on the Monday immediately after the season end. Then do the same for subsequent rounds.
Again, I would recommend doing some dummy simulations where you can play around and test things before running it for real on your league. And the main forum may be able to provide further advice on particular details.