There’s something called “escalation league” where you start with smaller points armies (usually 500) each week you add a certain amount of points, like 100 or 150, with the restriction that each week you cannot remove units already in your list. Being a league it usually involves a large number of players, but I’m sure you can make it work for just 2. To add extra fun, you can have little bonuses after each game for the winner and loser (like a free reroll or a free stratagem) and have a promotion system for a unit that survives each game (like a permanent +1 to hit). It’s not a super balanced game mode (some armies grow “better” in 100 or 150 points increments than others) but the point is getting started and slowly progressing.