Metabolism is a term usually thrown around too loosely.

basal metabolism (BMR) - for truly basic functions - when measured it's at the end of sleep cycle when body is done repairing whatever is going to be done, and still basically asleep.
They wake you in the lab, put a hood over your head, and let you go back to sleep.

You had RMR or resting metabolism tested - which includes being awake and brain is one of the bigger metabolic organs that accounts for most of your base burn, plus your other active systems - even though sitting in chair calmly resting. So it is higher than BMR, usually by 150-250 cal daily.

Some people apply metabolism to higher functions, to include eating or all day - but that's not really.

So a valid RMR test would be day after rest day - if body was under repair from prior workout still - falsely elevated RMR and bad test.
Same as if you ate within 4-5 hrs, because the digestion causes more CO2 to be expired and that fools the calculations for energy use.

But for sake of comment, lets say totally valid test results.
Your body can adjust other systems so the basic metabolism functions get the calories they need.
First thing a body does is slow down daily activity, Spontaneous Physical Activity (SPA), which would even include physical "ticks" like shaking leg, moving arms, getting up more frequently just to move, ect.
Or in your case - it never sped up to include what could be included. You likely have room for your TDEE to go even higher.
Or willing to burn more to keep warm.
Or grow hair/nails/skin at a normal rate - which you've likely not experienced for a very long time.

Metabolic efficiency is what it's called for other aspects. Body can be wasteful for even those base functions if enough calories around.

So while you could keep a higher RMR say than expected, your daily burn could still be much lower than normal.

Also, eating more and lifting is great way to get body to speed back up, so the fact of 20 years suppressed doesn't mean it would take longer to recover.
One study showed that even still eating at lab measured suppressed TDEE, body started speeding back up partially in 3 months. Most people find it speeds up faster eating more and lifting - which you did.

How do you know it wasn't losing fat during that time of eating 1800 while metabolism was increasing?
True, for some of that time likely not, but near end probably was.

Well, even if the 2400 was say 250 more than reality on average - it would take 2 weeks eating that daily to slowly gain 1 lb - if with your lifting - not even all fat.

Now women can usually gain 1 lb in a month at surplus level of eating of muscle mass.
So close to that figure.

But I'd still wait and see. At least no need to increase slowly waiting for metabolism to speed up. But you may need time for daily movement to increase to expected levels, even if that good fidgeting.

Well, slow at first is good so body knows the craziness isn't continuing, like 100 extra daily for a couple of weeks - but then 250 hops for a week is good after that.

Any water weight from increased carb storage will happen in first few weeks with just a fewer extra calories.
That in itself speeds up metabolism dealing with that stored water.
After that happens, the big jumps aren't so bad.

I'm sure with good lifting program, you already got a good recomp going and will continue to. May not need 6 months. I've never seen a study indicate that a longer time suppressed needed a longer time to recover.
Some bodies just genetically adapt fast to changes, but always seems to be the fastest is down compared to up, no matter what.

That's why the frequent break weeks are good, just in case you have a body wanting to adjust too fast.
Also why the average weekly TDEE method works so well - your literal deficit each day is variable, from perhaps some days being no deficit to other days bigger than 15%. But that keeps the body pliable (not guessing or shocked) as it should be.