From what I understand the TRX-1 doesn't pick up trunked systems on DMR or NXDN.
That's not exactly the case. The TRX-1, as well as other Whistler scanners that can receive DMR, don't actually follow DMR systems and trunk-track them, like is the case for P25 systems as an example. However, they do allow the entry of talkgroup IDs, and can be set to only stop and monitor the programmed TGIDs. If the system is not extremely large, or very busy, their method works reasonably well. The Uniden scanners do actually follow (trunk-track) a system if it is correctly set up, with the LCNs being used. Uniden pays a fee for each scanner upgraded for DMR, so the upgrade fee is in place to address the additional cost. As the Whistler scanners do not actually follow the system, no upgrade fee is being charged. The TRX-1 & TRX-2 can also receive NXDN, but again, they only receive it and can be programmed with specific talkgroups.
Though if I were to get the BCD436HP and upgrade to to the two systems, would it be better able to pick up them up? Thanks.
The 436HP, with the DMR and/or NXDN upgrades, would actually trunk track the system(s) correctly, whereas the TRX-1 will not. But, as already noted, you may find that the method the TRX-1 uses may be satisfactory to you. Hopefully, someone in your area that has the TRX-1 (or it's sibling the TRX-2) can comment how well it works for them. (I see Forts jumped in while I was typing.)
In my area, for the two NXDN public safety systems that I monitor, My TRX-1 will miss some calls that my 436HP or SDS100 catch.