eleven year old boxer developed new onset increased thirst and urination over the last six weeks, initial workup is normal so far, and im trying to figure out what the next tier of testing should look like before we spiral

Bruno is our 11.5 year old male boxer, neutered at 18 months, 68lb, generally in great shape for his age, no chronic conditions until now. we started tracking his water intake about 6 weeks ago because my partner noticed the water bowl was empty at odd times of day, and once we started measuring it turns out he is drinking somewhere between 2.4 and 2.8 liters per day, which for a 68lb dog is roughly double the expected 40-60ml per kg range. urination has scaled up to match, he is asking to go out roughly every 3 hours during the day and once overnight (he used to sleep through the night for the last 9 years, no exceptions), and the volume per event is clearly larger, not smaller, so this is not a UTI presentation. he is not straining, blood is not visible, and he is still fully continent.

appetite is unchanged, weight is stable at his usual 68lb (checked at home on our people scale, hes been within 1lb of that for two years), energy is a little lower than baseline but not dramatically so, and hes still doing his normal 45 minute morning walk without complaint. no vomiting, no diarrhea, coat looks the same, no visible lumps that we or our vet have found on the every 4 month senior physical we do.

the workup so far. first vet visit at week 2 of the increased drinking, they ran a basic senior panel (CBC, chem, T4, UA with sediment) and everything came back normal. specifically, kidney values were mid-range normal, glucose was normal, calcium was normal, T4 was normal, urine specific gravity was on the low end at 1.018 but not diagnostically dilute. she said to monitor for 3-4 weeks and recheck, which we did last friday. second run of the same panel plus a urine culture came back with the same picture, kidney values still normal, urine culture negative, specific gravity slightly lower at 1.014 this time but still not in the "definitely diluted" range. she then said the next tier would be either an ACTH stimulation test for Cushings or a low dose dex suppression, or alternately abdominal ultrasound to look at adrenal size and kidney architecture, and estimated the ACTH at about $340 and the ultrasound at about $525.

where im stuck. one, is the "workup is normal, watch and wait" position defensible for another 4-6 weeks or is that going to cost us diagnostic time on something like early Cushings where earlier detection matters. two, if we are going to spend the money on further diagnostics, which one do people with experience recommend as the higher yield first move for this specific presentation (11yo boxer, PU/PD, normal chem, normal T4, mildly dilute urine but not diagnostically so). three, are there any home observations we should be tracking that would either raise or lower the urgency, im already logging water intake daily but im not sure what else to be watching. four, boxer specific considerations, i know boxers have a higher baseline cancer rate at this age and part of me is worried this is the first symptom of something like a lymphoma or an adrenal tumor and were burning time on the cheap tests, but i also dont want to bankrupt us with a full body scan on a "probably nothing" workup. what would you do