I think you have made an important and valid point about availability of appropriately-sized aircraft for FRA and MUC, B_T.
You alluded to LH operating BRS-FRA themselves which they did for 13 months from the end of March 2008. They used Eurowings Bae 146-300 aircraft at 3 x daily (21 x weekly), including a night stopper. In that time they carried just under 100,000 passengers before ending the route citing the recession that was at its height. 2009 is the only year this century that has seen a drop in BRS's overall annual passenger numbers.
3 x daily was very good for connectivity at FRA but at that frequency even the Bae 146-300 was probably too large. Whether it would have succeeded in more promising economic times is hard to gauge.
In 2013 bmi regional (as flybmi then was) set up a Bristol base and began a number of routes, with Germany seeing Frankfurt, Munich, Hamburg and Hanover - the latter lasted only a year but Dusseldorf began in 2015.
With Frankfurt rising to 3 x daily on weekdays and Munich double-daily, annual passenger figures rose between 2013 and 2018 from 16,000 to 53,000 and from 10,000 to 48,000 respectively. The LH code shares undoubtedly played a significant part.
Prior to Eurowings, BA operated both FRA and MUC from the beginning of this century with small E-Jets, first as Citiexpress then as Connect. It was a single daily service on both although FRA increased to 2 x daily at one point. So far as I know both were P2P. In 2007 Flybe bought BAConnect and promptly closed the five-aircraft Bristol base and with that went FRA and MUC, plus DUS that had commenced the previous year, along with several non-German routes. With flybmi's situation it's almost like history repeating itself, except that the reasons are different.
So from having six German routes until the middle of February BRS now has only two, both year round - easyJet to Berlin SXF mainly daily in summer, slightly less in winter, and Ryanair to Cologne at mainly 4 x weekly.
Although in terms of BRS's overall numbers the loss of the flybmi passengers won't create a huge dent, the German routes were more important than the passenger numbers lost. BRS needs more hub connectivity, especially with BRU's immediate future still seemingly unclear albeit the route continues to remain available for booking from 1 May on the Brussels Airlines website, and of course Germany is a major European country with which most regional airports would want as much connectivity as possible.