I'll say up front that I have mixed feelings about ABA and I think it's easy to go wrong (though also: we all use ABA in our daily lives - every time we use a Fitbit or promise ourselves a coffee when we've finished the cleaning - so it's also easy to demonise). The history of ABA is pretty horrible so obviously creates a lot of fear and concern. So if you do go that route it's worth reading up about the concerns so you know what to watch out for and avoid.
But having said that early on when we were feeling really lost we did distance ABA and found it very helpful. Really it was a hand hold and they suggested lots of different parenting techniques - it wasn't purely ABA focused. E.g. we did a lot of modelling, co-regulating, backward chaining, breaking tasks into small steps, talking about feelings/how to process emotions that was really helpful but not about behavioural modification at all. We also did some more "ABA-ish" things like sticker charts, which I think were sometimes useful though they weren't motivating for very long (actually we found the same with our NT son: sticker charts worked for a few months around 2.5-3 years old and then were completely boring and non-motivating!)
We used a company called Beam ABA and did two calls a month with the tutor (so after their initial session with DD they weren't hands on with her at all, we were doing all the "therapy"). Financially that was much more manageable. I've checked the package we did and it's now £250/month, so not cheap still, but much less than having therapists in your house. I also felt much safer doing it myself and being in full control of the process. We did it for a couple of years (maybe from around age 3 to 5?) By the end we were sort of tutoring ourselves (we'd come to the calls with a list of ideas/suggestions and the tutor would say "that sounds good" :) ). We probably could have skipped the last year without missing anything much, but we really appreciated the hand holding while we were doing the transition to school and while we got into new routines.
It is very early days though, and I do think the most valuable thing you can do is just play with them and follow their lead on what they seem interested in (even if that's something "autistic-seeming" like spinning or pouring or whatever). The point is that it still teaches your children that interacting with you is fun and valuable if you can help them to do what they are already motivated to do.
There is a great Facebook group called "Nurturing Neurodiversity" that you might like. A lot of parents there are still pre-diagnosis and will know how hard that limbo stage is. The lady who runs it also has some good videos on YouTube. Here is an example one, talking about things you can do that aren't "therapy" (but could be run alongside speech therapy or occupational therapy or anything else you might choose to do).
She has a bunch of different videos that might be useful later too. A mix of very practical subjects and the more emotional side of things.
It's also worth checking your local diagnostic pathway (i.e. how you would get an autism diagnosis later if it turns out that is what's going on), just so you can make sure you are in the right process as soon as possible. There may also be services (e.g. Portage) that you can access pre-diagnosis. If you Google your local authority name and "Local Offer" you should find a page with lots of information on special needs services, local support groups, etc. Some of it will be restricted to children who already have a diagnosis, but hopefully some of it will be available without a diagnosis too - and it should also give you some idea of how the diagnostic pathway works in your area. For example, here is a leaflet that shows how it works in my area: but annoyingly every area is different (might give you some ideas of search terms to try to find information for your area though!): cityandhackneycamhs.org.uk/wp-content/uploads/2015/10/ASD-assessment-leaflet-2016-pdf.pdf
As previous posters said, having an outside professional on your side can really help, so even some very part-time nursery might have value if the nursery could then help explain and validate your concerns. It was childcare who first picked up our DD's issues, and when she was formally assessed at nearly 4 a visit to her at her nursery (and talking to her nursery teachers) was an important part of the assessment. She also loved nursery and I think it made the transition to school much easier for her when the time came (but obviously you'll get your funded hours well before that, so no rush if finances won't allow it).
It's tricky - because in terms of best use of funds, you may find that topping up nursery costs when they're a bit older is actually more "bang for your buck" than throwing money at therapists now. But I remember very well how horrible this stage of having concerns but not really knowing what to do about them is, so I know the temptation to do "anything available" is very strong!