Is it permitted to stop in the middle of a brocha in shemoneh esrei to think one’s requests and reflect on the meaning of the words etc. in order to arouse kavanna? Is this considered a hefsek? If it is allowed, are there any constraints on what one is allowed to think or how long one can think for (like there are when the requests are verbalised)?
Yes, it is allowed (1).
It is hard to give exact guidelines as to what and how long you can reflect on the beracha. There is a very nice sefer called “Pathways to Prayer” that might shed some light on different and appropriate thoughts (kavana) that a person should have during davening.
All the best
(1) See, for example, Yesod Ve’shoresh Ha’avodah shaar 5 perek 3-4.