Following a 'rall. poco a poco' in the previous phrase, ending in a fermata pause, I wish to indicate that the new phrase is slightly faster than 'a tempo'. I had initially thought 'a tempo con moto' would be appropriate, however I haven't seen a marking like this in music so I am unsure. Would 'a tempo con moto' make sense and convey what I want?
I think I'd understand what you meant, but it's a little weird. "Con moto" is typically attached to a tempo marking like "Allegro". I think that "A tempo, piu mosso" would work.
Alternatively, there's no reason you have to stick to the Italian markings nowadays. You can simply write "faster than before" or something like that.
'A tempo but slightly faster'. Or' if you're obsessed about doing it in Italian, 'a tempo ma più mosso'.
No need for 'poco a poco' after 'rall'. 'Rall.' means 'gradually getting slower'. If you're using notation software and care about playback, be sure to extend the dotted line. Likewise, don't expect Sibelius etc. to recognise 'a tempo ma più mosso'. Turn off playback for the text object and add a hidden mm mark.