Are we setting up a bad habit for our toddler?

My husband and I have a wonderful almost 16 month old boy. He was never a great sleeper as a baby so we tried Ferber at 6 months to vague success, but this backtracked at about 7 months. We sleep trained again (again with the Ferber method) at about 10 months and this is what stuck - since then our son has slept in his own room and sleeps through the night.

However, it dawned on my recently that perhaps this is no longer considered sleeping through the night, and I would greatly appreciate any input as to how/if we can improve our current situation, especially as we have a baby girl due in 6 weeks' time.

Until a month or so ago, our son's bedtime was consistently 7:30pm (+/- 15 mins) and he would sleep until 6:30am. This worked well for us and felt like a dream, especially since he was sticking to this pretty much every night. However that wake up time has crept earlier and earlier. When he started waking up regularly before 6am, our go-to plan of action was to bring him into our bed (for ease), give him his morning bottle of milk, and then we would all try to go back to sleep for as long as possible.

This worked reasonably well, but the wake up time kept getting earlier. We live in the UK so when the clocks went forward at the end of March, the sun started rising closer to 4am. We've had a lot going on but will have blackout curtains put up in his room within the next week. Hopefully this will help with some of the problem?

What we have been doing for the past few weeks is giving our son his morning bottle in his room, then immediately putting him back down in his cot. This works well and we often get another 2-3 hours of sleep out of him after a 4am wake up. While we're happy with the current setup, our concern is that this is setting up a bad habit for our toddler as it essentially amounts to a night feed when we know he does have the ability to sleep through the night.

Should we be re- sleep training for this morning wake up? Should we be giving water instead of milk? Should we try using a grow clock or something similar? Any advice or suggestions are welcome!

For additional context: He eats well and although he has some milk from an open cup during the day (at snack/mealtimes), the 7oz morning bottle is his only bottle of the day. He started walking at 15 months which we thought might have caused this regression, but it doesn't seem to be waning even now. He has all four of his pre-molars but his canines are taking a while to come through. He is, most days, on one nap a day, though car journeys sometimes mean he naps twice. For a one nap day, this nap usually falls around midday for 1.5 hours or so. The super early (4am) wake ups don't seem to correspond specifically to 2-nap days or even days with a shorter final wake window. They seem to happen randomly. We've always felt that he was a fairly low-sleep needs child, sitting comfortably at about 11 hours of sleep at night and 1.5 hours during the day.

Thank you for reading!