I mean regular is subjective so lets take the last 5 years and 10 years as examples
In the last 5 years Bayern have brought 27 players from other clubs... of which 10 were from German clubs (so pretty much 37%)
In the last 5 years Bayern have spent 412.9 million on players of which 109.9 million was from german clubs (so pretty much 26.5%)
These 10 signings also included Gortexka, Rudy and Nubel who were actually a free agenst so you could even argue they technically didnt sign him from a german cub and in reality they signed 26% of players from german clubs
in terms of weakening the teams that are challenging them then this is who they signed them from
20/21 Nubel - Shalke - finished 18th
19/20 Cuissinance - Monchengladbach - finished 4th (17 points behind Bayern)
19/20 Arp - Hamberg - were in division 2
19/20 Pavard - Stutgart - were in division 2
18/19 Gortexka - Shalke - finished 14th
17/18 Sule - Hofenheim - Finished 3rd (29 points behind Bayern)
17/18 Wagner - Hofenheim - Finished (29 points behind Bayern)
17/18 Rudy - Hofenheim - Finished(29 points behind Bayern)
17/18 Gnabry - Werder - finished 16th
16/17 Hummles - Dortnund - finished 3rd (18 points behind Bayern)
I think the only one you could even make an argument they were weakening a club who were a threat is Hummles
1 signing out of 27 (about 4%) and 31.5 out of 412.9 million or about 7.5% of their spend
As i say regular is subjective and if 96% of their signings had conformed to your theory or even 92.5% of their spend I would agree with you - but as the opposit is true it seems a stretch to say this is regular
Opening this up to a 10 year window and we see
57 players brought from other clubs of which 24 were from german clubs so 42% - an increase but clearly still most players were not brought from german clubs
total spend over that period was 654.1 million with 202.6 of that being spent with German clubs - so around 30%
Players signed from clubs who could genuinley be considered rivals ober that perios would be
Lewendowski (though he is a free agent so you could argue shouldnt count) and Gotze
which means over a 10 year period we have 2 or 3 players out of 57 (so between 3.5 and 5.25% depending how you could lewendowski) players signed from a genuine BL rival and 9.8% of money spend on those
So 95% of signings and 90% of money spent falls outside of this supposedly regular occurance
i mean totally 100% feel free to show me some evidence from the last 10 years that contradicts this
https://www.transfermarkt.co.uk/fc-bayern-munchen/alletransfers/verein/27