How to expand peacefully as a HRE OPM or near?
When you claim the throne of a presumptive conquest, you are dissuaded by big powers guaranteeing their independence or allied with them, and low legitimacy small countries have always royal marriages with powerful countries.
Truly peacefull can only be trough diplo-annexing or colonization, barring a very very very lucky inheritance.
Warlike, Its a question of waiting for the right time and then committing fully. Make use of allies and wait for a chance. This can come soon or late.
Example: Taking on Burgundy would be suicide. But Burgundy gets into its inevatible war with France at some point. Usually they have the emperor and allies as backup and win. However if the emperor is out for some reason joining in could tip the balance. Take some provinces and release Bar and you have divided Burgundy in 2 making a small threat out of a big threat and making gains. If you have France as ally and managed to take Antwerp you have effictivly gutted the burgs. unlawfukll or not.
If you are playing a rich OPM you can also choose to expand in turkey against the non ottomans. If the Ottoturks attack you, you have the emperore as back up. Either you can win or you don't commit and wait for the emperor to be somewhere in asia minor/greece before you start a war in the HRE.
While you are waiting for a chance, build up cash. Its boring but it works. As Norway I had a forcelimit of 9. the swedes had an army of 22k. Taking out 5 loans I was able to hire enough mercs to outnumber them and win the war with my 6 shock general (get an advisor for army tradition while you wait for your chance). But my inflation ran up into the 23%'s. Was losing 37 gold a year with in income of about 18 a year Took untill the 1700's to get back to 0 inflation. A healthy heap of gold helps here