Hi and welcome to the forum,
I just tried this on Fairphone 5, a pair of ATH-M50XBT2’s and LDAC codec with 990 kbit/s bitrate. Didn’t hear a noticeable difference to the default variable bitrate to be honest, but everything worked fine, so it’s not incapable processor that’s causing this.
TLDR; My hypothesis is that you’re using different Bluetooth codec than before and that can’t output as high bitrates reliably as the old one. The good news is it shouldn’t have to, so do feel free to turn the bitrate down to either automatic or what gives you a stable listening experience to listen to high quality, stutterless audio, as it really shouldn’t matter that much. If you were happier with the listening experience before, change your Bluetooth codec to LDAC (see below for more details).
Here's the answer to audio nerds in more detail:
I searched a bit and it seems the headphones you’re referring to support new Bluetooth LE audio codecs LC3. It’s much better than the old SBC, but doesn’t work with your old A52S, as it only has Bluetooth 5.0 and for Bluetooth LE audio you need Bluetooth 5.2 or newer, which Fairphone 5 has.
The new Bluetooth LE standard doesn’t, however, support as high bitrates as the old one. Instead it uses better compression algorithms to keep the audio sounding good, similar to Qualcomm’s proprietary Aptx algorithms. Bluetooth claims that with Bluetooth LE audio codecs such as LC3 you can have better audio quality with less than half the bitrate in comparison to old Bluetooth classic standard SBC.
If you, however, try to manually change the Bluetooth LE Bitrate to 990 kbit/s, you’ll probably run in to audio stuttering. The maximum theoretical bitrate Bluetooth LE supports is around 1300 kbit/s in comparison to 2100 kbit/s Bluetooth classic supports. That means if you have any background radiation or there’s some obstacle between your phone and the headphones (such as phone being in your pocket), it won’t work well with that high bitrate.
Here’s a quick article about the Bluetooth LE codec. See also the linked video in the article explaining more about the bitrate limits.
If you, for some reason want to have the 990 kbit/s streaming work, just change the audio codec to LDAC (your headphones seem to have that too) and keep your phone close to the headphones. You’ll get a reduced battery life but more bits 
If you try changing the codec, please note there’s been some controversy with that:
LDAC not available, no automatic connection to headphones