search on the forum for eargo 5 review, i wrote one up last year. same situation as yourself- phonak p90s and needed something more discreet. so bought eargo 5 off ebay.
you will be able to activate them/use them without any problems. you just download the app, do the insitu audio test and then it programs the aids. you can adjust basics afterwards - volume, treble, bass and set those changes to permanent.
what i found off ebay and the eargo units. the batteries are the only questionable thing about the units. i have one unit that goes into low battery after about 8 hours. so i end up doing a charge during the day. it is not an issue for me, as i frequently have to take them out anyway. but if i wanted perfect aid or to have them repaired, eargo would not do that. you are stuck.
would i buy another set again off ebay. yes. do i like the eargos, yes. i use them 90% of the time. everyday. they are much nicer for me than the 90s. i especially like the lack of ‘hair bristle’ or ‘wind bristle’ that you get with the 90s (or any bte aid since the mics are on the back). the invisible in the ear aids (eargo) use your natural ear shape to amplify/locate sounds. you are not relying on mics that are sitting behind or on top of your ears. very similar to the Lyric aids in that regard.
good luck