I have some of
this type, and some of
this type, of stick-on thermometers... Both have the temperature numbers on them, that indicate (color change) where the fermenting brew/wort is at for temperature. Pretty easy to read and understand, no real explanation required. If you don't have either of those, get some. At <$3 each, it's a really cheap way to know what temperature the wort is fermenting at.
You can also find others on Amazon in different temperature ranges. I picked up some for my kegs, so that I'll be able to see what the brew temp is without getting a more expensive thermometer with probe.
Your brew shouldn't change that radically over just a few weeks. Unless you're storing it HOT.
Personally, I leave the brew in primary until it's done/ready for bottle/keg. IF I'm going to age it with something else, I do so once it's otherwise ready for bottle/keg. IMO, racking to secondary before then is unnecessary for 95% (or more) of the styles being made. When I do rack, it's because the flavor/aging element does a much better job when off the yeast. OR, I'm racking off of another element onto a secondary element where I need to halt the addition of the previous.
How are you oxygenating your wort? If just the shake (or pour) method, look to get an actual O2 infusion setup.
BTW, many [Wyeast] ale yeasts don't like the temperatures over the low (to mid) 70F area (70-75F being the ceiling on them). Since fermenting wort can be more than a few degrees above ambient temp, you could [easily] be fermenting too warm. Without a thermometer to tell you what the wort is fermenting at, you'll never actually know. I ferment where the ambient is in the 62-65F year round (a basement). This is a confirmed range (not just guesswork). This also has my brews ferment, roughly, in the middle of their temperature range. I have excellent brews months after being bottled.
I would suggest eliminating as many variables as you can. Measure the priming sugar by weight. Try a few long primary, no secondary batches (get more primaries if you must). Oxygenate with pure O2 via a stone. Confirm the actual wort temperature while fermenting.